title: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
body: <p>I'm new to this and have the following error when trying to add a Navigation Drawer to an activity with Fragments:</p>
  
  <pre><code>java.lang.RuntimeException: Unable to start activity ComponentInfo{uk.co.XXXX.EditFactFind}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
          at android.app.ActivityThread.access$800(ActivityThread.java:151)
          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
          at android.os.Handler.dispatchMessage(Handler.java:102)
          at android.os.Looper.loop(Looper.java:135)
          at android.app.ActivityThread.main(ActivityThread.java:5257)
          at java.lang.reflect.Method.invoke(Native Method)
          at java.lang.reflect.Method.invoke(Method.java:372)
          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
          at uk.co.XXXX.NavigationDrawerFragment.setUp(NavigationDrawerFragment.java:135)
          at uk.co.XXXX.EditFactFind.onCreate(EditFactFind.java:69)
          at android.app.Activity.performCreate(Activity.java:5990)
          at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
  </code></pre>
  
  <p>My 1st Java file is:</p>
  
  <pre><code>package XXXX;
  
  import android.support.v4.app.FragmentActivity;
  import android.support.v7.app.AppCompatActivity;
  import android.app.Activity;
  import android.app.ActionBar;
  import android.support.v4.app.Fragment;
  import android.support.v4.app.ActionBarDrawerToggle;
  import android.support.v4.view.GravityCompat;
  import android.support.v4.widget.DrawerLayout;
  import android.content.SharedPreferences;
  import android.content.res.Configuration;
  import android.os.Bundle;
  import android.preference.PreferenceManager;
  import android.view.LayoutInflater;
  import android.view.Menu;
  import android.view.MenuInflater;
  import android.view.MenuItem;
  import android.view.View;
  import android.view.ViewGroup;
  import android.widget.AdapterView;
  import android.widget.ArrayAdapter;
  import android.widget.ListView;
  import android.widget.Toast;
  
  /**
   * Fragment used for managing interactions for and presentation of a navigation drawer.
   * See the &lt;a href="https://developer.android.com/design/patterns/navigation-drawer.html#Interaction"&gt;
   * design guidelines&lt;/a&gt; for a complete explanation of the behaviors implemented here.
   */
  public class NavigationDrawerFragment extends Fragment {
  
      /**
       * Remember the position of the selected item.
       */
      private static final String STATE_SELECTED_POSITION = "selected_navigation_drawer_position";
  
      /**
       * Per the design guidelines, you should show the drawer on launch until the user manually
       * expands it. This shared preference tracks this.
       */
      private static final String PREF_USER_LEARNED_DRAWER = "navigation_drawer_learned";
  
      /**
       * A pointer to the current callbacks instance (the Activity).
       */
      private NavigationDrawerCallbacks mCallbacks;
  
      /**
       * Helper component that ties the action bar to the navigation drawer.
       */
      private ActionBarDrawerToggle mDrawerToggle;
  
      private DrawerLayout mDrawerLayout;
      private ListView mDrawerListView;
      private View mFragmentContainerView;
  
      private int mCurrentSelectedPosition = 0;
      private boolean mFromSavedInstanceState;
      private boolean mUserLearnedDrawer;
  
      public NavigationDrawerFragment() {
      }
  
      @Override
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
  
          // Read in the flag indicating whether or not the user has demonstrated awareness of the
          // drawer. See PREF_USER_LEARNED_DRAWER for details.
          SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(getActivity());
          mUserLearnedDrawer = sp.getBoolean(PREF_USER_LEARNED_DRAWER, false);
  
          if (savedInstanceState != null) {
              mCurrentSelectedPosition = savedInstanceState.getInt(STATE_SELECTED_POSITION);
              mFromSavedInstanceState = true;
          }
  
          // Select either the default item (0) or the last selected item.
          selectItem(mCurrentSelectedPosition);
      }
  
      @Override
      public void onActivityCreated(Bundle savedInstanceState) {
          super.onActivityCreated(savedInstanceState);
          // Indicate that this fragment would like to influence the set of actions in the action bar.
          setHasOptionsMenu(true);
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                               Bundle savedInstanceState) {
          mDrawerListView = (ListView) inflater.inflate(
                  R.layout.fragment_navigation_drawer, container, false);
          mDrawerListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
              @Override
              public void onItemClick(AdapterView&lt;?&gt; parent, View view, int position, long id) {
                  selectItem(position);
              }
          });
          mDrawerListView.setAdapter(new ArrayAdapter&lt;String&gt;(
                  getActivity(),
                  android.R.layout.simple_list_item_activated_1,
                  android.R.id.text1,
                  new String[]{
                          getString(R.string.title_section1),
                          getString(R.string.title_section2),
                          getString(R.string.title_section3),
                          "Assets",
                          "ID",
                  }));
          mDrawerListView.setItemChecked(mCurrentSelectedPosition, true);
          return mDrawerListView;
      }
  
      public boolean isDrawerOpen() {
          return mDrawerLayout != null &amp;&amp; mDrawerLayout.isDrawerOpen(mFragmentContainerView);
      }
  
      /**
       * Users of this fragment must call this method to set up the navigation drawer interactions.
       *
       * @param fragmentId   The android:id of this fragment in its activity's layout.
       * @param drawerLayout The DrawerLayout containing this fragment's UI.
       */
      public void setUp(int fragmentId, DrawerLayout drawerLayout) {
          mFragmentContainerView = getActivity().findViewById(fragmentId);
          mDrawerLayout = drawerLayout;
  
          // set a custom shadow that overlays the main content when the drawer opens
  //        mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);
          // set up the drawer's list view with items and click listener
  
          ActionBar actionBar = getActionBar();
          actionBar.setDisplayHomeAsUpEnabled(true);
          actionBar.setHomeButtonEnabled(true);
  
          // ActionBarDrawerToggle ties together the the proper interactions
          // between the navigation drawer and the action bar app icon.
          mDrawerToggle = new ActionBarDrawerToggle(
                  getActivity(),                    /* host Activity */
                  mDrawerLayout,                    /* DrawerLayout object */
                  R.drawable.ic_drawer,             /* nav drawer image to replace 'Up' caret */
                  R.string.navigation_drawer_open,  /* "open drawer" description for accessibility */
                  R.string.navigation_drawer_close  /* "close drawer" description for accessibility */
          ) {
              @Override
              public void onDrawerClosed(View drawerView) {
                  super.onDrawerClosed(drawerView);
                  if (!isAdded()) {
                      return;
                  }
  
                  getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
              }
  
              @Override
              public void onDrawerOpened(View drawerView) {
                  super.onDrawerOpened(drawerView);
                  if (!isAdded()) {
                      return;
                  }
  
                  if (!mUserLearnedDrawer) {
                      // The user manually opened the drawer; store this flag to prevent auto-showing
                      // the navigation drawer automatically in the future.
                      mUserLearnedDrawer = true;
                      SharedPreferences sp = PreferenceManager
                              .getDefaultSharedPreferences(getActivity());
                      sp.edit().putBoolean(PREF_USER_LEARNED_DRAWER, true).apply();
                  }
  
                  getActivity().supportInvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
              }
          };
  
          // If the user hasn't 'learned' about the drawer, open it to introduce them to the drawer,
          // per the navigation drawer design guidelines.
          if (!mUserLearnedDrawer &amp;&amp; !mFromSavedInstanceState) {
              mDrawerLayout.openDrawer(mFragmentContainerView);
          }
  
          // Defer code dependent on restoration of previous instance state.
          mDrawerLayout.post(new Runnable() {
              @Override
              public void run() {
                  mDrawerToggle.syncState();
              }
          });
  
          mDrawerLayout.setDrawerListener(mDrawerToggle);
      }
  
      private void selectItem(int position) {
          mCurrentSelectedPosition = position;
          if (mDrawerListView != null) {
              mDrawerListView.setItemChecked(position, true);
          }
          if (mDrawerLayout != null) {
              mDrawerLayout.closeDrawer(mFragmentContainerView);
          }
          if (mCallbacks != null) {
              mCallbacks.onNavigationDrawerItemSelected(position);
          }
      }
  
      @Override
      public void onAttach(Activity activity) {
          super.onAttach(activity);
          try {
              mCallbacks = (NavigationDrawerCallbacks) activity;
          } catch (ClassCastException e) {
              throw new ClassCastException("Activity must implement NavigationDrawerCallbacks.");
          }
      }
  
      @Override
      public void onDetach() {
          super.onDetach();
          mCallbacks = null;
      }
  
      @Override
      public void onSaveInstanceState(Bundle outState) {
          super.onSaveInstanceState(outState);
          outState.putInt(STATE_SELECTED_POSITION, mCurrentSelectedPosition);
      }
  
      @Override
      public void onConfigurationChanged(Configuration newConfig) {
          super.onConfigurationChanged(newConfig);
          // Forward the new configuration the drawer toggle component.
          mDrawerToggle.onConfigurationChanged(newConfig);
      }
  
      @Override
      public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
          // If the drawer is open, show the global app actions in the action bar. See also
          // showGlobalContextActionBar, which controls the top-left area of the action bar.
          if (mDrawerLayout != null &amp;&amp; isDrawerOpen()) {
              inflater.inflate(R.menu.global, menu);
              showGlobalContextActionBar();
          }
          super.onCreateOptionsMenu(menu, inflater);
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          if (mDrawerToggle.onOptionsItemSelected(item)) {
              return true;
          }
  
          if (item.getItemId() == R.id.action_example) {
              Toast.makeText(getActivity(), "Example action.", Toast.LENGTH_SHORT).show();
              return true;
          }
  
          return super.onOptionsItemSelected(item);
      }
  
      /**
       * Per the navigation drawer design guidelines, updates the action bar to show the global app
       * 'context', rather than just what's in the current screen.
       */
      private void showGlobalContextActionBar() {
          ActionBar actionBar = getActionBar();
          actionBar.setDisplayShowTitleEnabled(true);
          actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
          actionBar.setTitle(R.string.app_name);
      }
  
      private android.app.ActionBar getActionBar() {
           return ((FragmentActivity) getActivity()).getActionBar();
  
      }
  
      /**
       * Callbacks interface that all activities using this fragment must implement.
       */
      public static interface NavigationDrawerCallbacks {
          /**
           * Called when an item in the navigation drawer is selected.
           */
          void onNavigationDrawerItemSelected(int position);
      }
  }
  </code></pre>
  
  <p>My 2nd is:</p>
  
  <pre><code>package XXXX;
  
  
  import java.io.Serializable;
  import java.text.DateFormat;
  import java.text.SimpleDateFormat;
  import java.util.Calendar;
  
  
  import android.app.AlertDialog;
  import android.app.FragmentManager;
  import android.app.FragmentTransaction;
  import android.content.DialogInterface;
  import android.content.Intent;
  import android.os.Bundle;
  import android.support.v4.app.FragmentActivity;
  import android.support.v4.widget.DrawerLayout;
  import android.app.ActionBar;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.view.View;
  import android.view.View.OnClickListener;
  import android.widget.Button;
  import android.widget.EditText;
  import android.widget.TextView;
  
  public class EditFactFind extends FragmentActivity implements NavigationDrawerFragment.NavigationDrawerCallbacks {
  
      public static final int RESULT_DELETE = -500;
      private boolean isInEditMode = true;
      private boolean isAddingFactFind = true;
      private NavigationDrawerFragment mNavigationDrawerFragment;
      private CharSequence mTitle;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.edit_factfind);
  
          final Button saveButton = (Button)findViewById(R.id.saveButton);
          final Button cancelButton = (Button)findViewById(R.id.cancelButton);
          final EditText titleEditText = (EditText)findViewById(R.id.titleEditText);
          //final EditText factFindEditText = (EditText)findViewById(R.id.factFindEditText);
          final TextView dateTextView = (TextView)findViewById(R.id.dateTextView);
          final Button nextButton =(Button) findViewById(R.id.nextButton);
  
          //Create fragment and give it an argument for the selected article
          secA_pg1 iniSecFrag = new secA_pg1();
          Bundle args = new Bundle();
          args.putInt(secA_pg1.ARG_INDEX, 1);
          iniSecFrag.setArguments(args);
  
          FragmentTransaction initialTransaction = getFragmentManager().beginTransaction();
  
          // Replace whatever is in the fragment_container view with this fragment,
          // and add the transaction to the back stack so the user can navigate back
          initialTransaction.replace(R.id.fragment_container, iniSecFrag);
          initialTransaction.addToBackStack(null);
  
          //Commit the transaction
          initialTransaction.commit();
  
          //// TODO: 10/09/2015  
          mNavigationDrawerFragment = (NavigationDrawerFragment)
                  getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
          mTitle = getTitle();
  
          // Set up the drawer.
          mNavigationDrawerFragment.setUp(
                  R.id.navigation_drawer,
                  (DrawerLayout) findViewById(R.id.drawer_layout));
  
          Serializable extra = getIntent().getSerializableExtra("FactFind");
          if(extra != null)
          {
              FactFind factFind = (FactFind) extra;
              titleEditText.setText(factFind.getTitle());
            //  factFindEditText.setText(factFind.getFactFindTitle());
  
              DateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss");
              String date = dateFormat.format(factFind.getDate());
  
              dateTextView.setText(date);
  
              isInEditMode = false;
              titleEditText.setEnabled(false);
            //  factFindEditText.setEnabled(false);
              saveButton.setText("Edit");
  
              isAddingFactFind = false;
  
          }
  
          cancelButton.setOnClickListener(new OnClickListener() {
  
              @Override
              public void onClick(View v) {
                  setResult(RESULT_CANCELED, new Intent());
                  finish();
              }
          });
  
          nextButton.setOnClickListener(new OnClickListener() {
              @Override
              public void onClick(View v) {
                  //secA_pg1 secFrag = (secA_pg1) getFragmentManager().findFragmentById(R.id.Sec_A_pg1_fragment);
                  //Create fragment and give it an argument for the selected article
                  secA_pg2 newSecFrag = new secA_pg2();
                  Bundle args = new Bundle();
                  args.putInt(secA_pg2.ARG_INDEX, 2);
                  newSecFrag.setArguments(args);
  
                  FragmentTransaction transaction = getFragmentManager().beginTransaction();
  
                  // Replace whatever is in the fragment_container view with this fragment,
                  // and add the transaction to the back stack so the user can navigate back
                  transaction.replace(R.id.fragment_container, newSecFrag);
                  transaction.addToBackStack(null);
  
                  //Commit the transaction
                  transaction.commit();
              }
          });
  
          saveButton.setOnClickListener(new OnClickListener() {
  
              @Override
              public void onClick(View v) {
  
  
                  if(isInEditMode)
                  {
                      Intent returnIntent = new Intent();
                      FactFind factFind = new FactFind(titleEditText.getText().toString(),Calendar.getInstance().getTime());
                      returnIntent.putExtra("FactFind", factFind);
                      setResult(RESULT_OK, returnIntent);
                      finish();
  
                  }
                  else
                  {
                      isInEditMode = true;
                      saveButton.setText("Save");
                      titleEditText.setEnabled(true);
                     // factFindEditText.setEnabled(true);
                  }
  
              }
          });
      }
  
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          AlertDialog.Builder builder = new AlertDialog.Builder(this);
          builder.setMessage(R.string.are_you_sure_you_want_to_delete_this_fact_find_it_can_t_be_undone_);
          builder.setTitle("Confirm Delete");
          builder.setPositiveButton("Delete", new DialogInterface.OnClickListener() {
  
              @Override
              public void onClick(DialogInterface dialog, int which) {
                  Intent returnIntent = new Intent();
  
                  setResult(RESULT_DELETE, returnIntent);
                  finish();
  
              }
          });
          builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
  
              @Override
              public void onClick(DialogInterface dialog, int which) {
                  // TODO Auto-generated method stub
              }
          });
  
          builder.create().show();
          return true;
  
      }
  
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
  
          // Inflate the menu; this adds items to the action bar if it is present.
          if (!mNavigationDrawerFragment.isDrawerOpen()) {
              // Only show items in the action bar relevant to this screen
              // if the drawer is not showing. Otherwise, let the drawer
              // decide what to show in the action bar.
              getMenuInflater().inflate(R.menu.nav_drawer, menu);
              restoreActionBar();
              return true;
          }
          else {
              //return super.onCreateOptionsMenu(menu);
              getMenuInflater().inflate(R.menu.action_menu, menu);
              return true;
          }
      }
      //@Override
      public void onSelectedFragChanged(int index) {
          FragmentManager fragmentManager = getFragmentManager();
          secA_pg1 secA_pg1 = (secA_pg1) fragmentManager.findFragmentById(R.id.Sec_A_pg1_fragment);
  
          secA_pg1.setSectionTitle(index);
      }
  
      public void restoreActionBar() {
          ActionBar actionBar = getActionBar();
          //actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
          actionBar.setDisplayShowTitleEnabled(true);
          actionBar.setTitle(mTitle);
      }
  
      @Override
      public void onNavigationDrawerItemSelected(int position) {
  
      }
  }
  </code></pre>
  
  <p>I read through a few other articles that mentioned the ActionBar needs to be SupportActionBar but that threw up even more problems when I changed the imports over on both Java files. Quite possible that I've made a mistake there as well though.</p>