title: Android DrawerLayout with NullPointerException
body: <p>I'm getting stuck with this NullPointerException. I've searched similar posts and tried all of them, but none works. It seems like I'm missing something but I can't find what it is. Please help.</p>
  
  <p>Here is the Debug output:</p>
  
  <pre><code>E/AndroidRuntime: FATAL EXCEPTION: main
                Process: project_name, PID: 4101
                java.lang.RuntimeException: Unable to start activity ComponentInfo{project_name/project_name.activity.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.support.design.widget.NavigationView.getHeaderView(int)' on a null object reference
                    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                    at android.app.ActivityThread.-wrap11(ActivityThread.java)
                    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                    at android.os.Handler.dispatchMessage(Handler.java:102)
                    at android.os.Looper.loop(Looper.java:148)
                    at android.app.ActivityThread.main(ActivityThread.java:5417)
                    at java.lang.reflect.Method.invoke(Native Method)
                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.support.design.widget.NavigationView.getHeaderView(int)' on a null object reference
                    at project_name.activity.HomeActivity.onCreate(HomeActivity.java:93)
                    at android.app.Activity.performCreate(Activity.java:6237)
                    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                    at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                    at android.os.Handler.dispatchMessage(Handler.java:102) 
                    at android.os.Looper.loop(Looper.java:148) 
                    at android.app.ActivityThread.main(ActivityThread.java:5417) 
                    at java.lang.reflect.Method.invoke(Native Method) 
                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
  com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
  </code></pre>
  
  <p> 
  My Activities:</p>
  
  <pre><code>package project_name.activity;
  
  import android.content.Intent;
  import android.os.Bundle;
  import android.os.Handler;
  import android.support.design.widget.FloatingActionButton;
  import android.support.design.widget.NavigationView;
  import android.support.design.widget.Snackbar;
  import android.support.v4.app.Fragment;
  import android.support.v4.app.FragmentTransaction;
  import android.support.v4.view.GravityCompat;
  import android.support.v4.widget.DrawerLayout;
  import android.support.v7.app.ActionBarDrawerToggle;
  import android.support.v7.app.AppCompatActivity;
  import android.support.v7.widget.Toolbar;
  import android.view.LayoutInflater;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.view.View;
  import android.widget.ImageView;
  import android.widget.TextView;
  import android.widget.Toast;
  
  import com.bumptech.glide.Glide;
  import com.bumptech.glide.load.engine.DiskCacheStrategy;
  
  import project_name.activity.R;
  import project_name.activity.fragment.DashboardFragment;
  import project_name.activity.fragment.PhotosFragment;
  import project_name.activity.other.CircleTransform;
  
  public class HomeActivity extends AppCompatActivity {
  
      private NavigationView navigationView;
      private DrawerLayout drawer;
      private View navHeader;
      private ImageView imgNavHeaderBg, imgProfile;
      private TextView txtName, txtWebsite;
      private Toolbar toolbar;
      private FloatingActionButton fab;
  
      // urls to load navigation header background image
      // and profile image
      private static final String urlNavHeaderBg = "http://api.androidhive.info/images/nav-menu-header-bg.jpg";
      private static final String urlProfileImg = "https://lh3.googleusercontent.com/eCtE_G34M9ygdkmOpYvCag1vBARCmZwnVS6rS5t4JLzJ6QgQSBquM0nuTsCpLhYbKljoyS-txg";
  
      // index to identify current nav menu item
      public static int navItemIndex = 0;
  
      // tags used to attach the fragments
      private static final String TAG_HOME = "home";
      private static final String TAG_PHOTOS = "photos";
      private static final String TAG_MOVIES = "movies";
      private static final String TAG_NOTIFICATIONS = "notifications";
      private static final String TAG_SETTINGS = "settings";
      public static String CURRENT_TAG = TAG_HOME;
  
      // toolbar titles respected to selected nav menu item
      private String[] activityTitles;
  
      // flag to load home fragment when user presses back key
      private boolean shouldLoadHomeFragOnBackPress = true;
      private Handler mHandler;
  
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
          toolbar = (Toolbar) findViewById(R.id.toolbar);
          setSupportActionBar(toolbar);
  
          mHandler = new Handler();
  
          drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
          navigationView = (NavigationView) findViewById(R.id.nav_view);
          fab = (FloatingActionButton) findViewById(R.id.fab);
  
          // Workarounds with Null Pointer Exception
  //        View headerLayout =
  //                navigationView.inflateHeaderView(R.layout.nav_header_home);
  //        navigationView = (NavigationView) headerLayout.findViewById(R.id.nav_view);
  //        navHeader = navigationView.inflateHeaderView(R.layout.nav_header_home);
  //        navHeader = LayoutInflater.from(this).inflate(R.layout.nav_header_home, null);
  //        navigationView.addHeaderView(navHeader);
  //        TextView text = (TextView) navHeader.findViewById(R.id.textView);
  //        text.setText("HELLO");
  //        navHeader = navigationView.inflateHeaderView(R.layout.nav_header_home);
  
          // Navigation view header
          navHeader = navigationView.getHeaderView(0);
          txtName = (TextView) navHeader.findViewById(R.id.name);
          txtWebsite = (TextView) navHeader.findViewById(R.id.website);
          imgNavHeaderBg = (ImageView) navHeader.findViewById(R.id.img_header_bg);
          imgProfile = (ImageView) navHeader.findViewById(R.id.img_profile);
  
          // load toolbar titles from string resources
          activityTitles = getResources().getStringArray(R.array.nav_item_activity_titles);
  
          fab.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View view) {
                  Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                          .setAction("Action", null).show();
              }
          });
  
          // load nav menu header data
          loadNavHeader();
  
          // initializing navigation menu
          setUpNavigationView();
  
          if (savedInstanceState == null) {
              navItemIndex = 0;
              CURRENT_TAG = TAG_HOME;
              loadHomeFragment();
          }
      }
  
      /***
       * Load navigation menu header information
       * like background image, profile image
       * name, website, notifications action view (dot)
       */
      private void loadNavHeader() {
          // name, website
          txtName.setText("My Name");
          txtWebsite.setText("project_name");
  
          // loading header background image
          Glide.with(this).load(urlNavHeaderBg)
                  .crossFade()
                  .diskCacheStrategy(DiskCacheStrategy.ALL)
                  .into(imgNavHeaderBg);
  
          // Loading profile image
          Glide.with(this).load(urlProfileImg)
                  .crossFade()
                  .thumbnail(0.5f)
                  .bitmapTransform(new CircleTransform(this))
                  .diskCacheStrategy(DiskCacheStrategy.ALL)
                  .into(imgProfile);
  
          // showing dot next to notifications label
          navigationView.getMenu().getItem(3).setActionView(R.layout.menu_dot);
      }
  
      /***
       * Returns respected fragment that user
       * selected from navigation menu
       */
      private void loadHomeFragment() {
          // selecting appropriate nav menu item
          selectNavMenu();
  
          // set toolbar title
          setToolbarTitle();
  
          // if user select the current navigation menu again, don't do anything
          // just close the navigation drawer
          if (getSupportFragmentManager().findFragmentByTag(CURRENT_TAG) != null) {
              drawer.closeDrawers();
  
              // show or hide the fab button
              toggleFab();
              return;
          }
  
          // Sometimes, when fragment has huge data, screen seems hanging
          // when switching between navigation menus
          // So using runnable, the fragment is loaded with cross fade effect
          // This effect can be seen in GMail app
          Runnable mPendingRunnable = new Runnable() {
              @Override
              public void run() {
                  // update the main content by replacing fragments
                  Fragment fragment = getHomeFragment();
                  FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
                  fragmentTransaction.setCustomAnimations(android.R.anim.fade_in,
                          android.R.anim.fade_out);
                  fragmentTransaction.replace(R.id.frame, fragment, CURRENT_TAG);
                  fragmentTransaction.commitAllowingStateLoss();
              }
          };
  
          // If mPendingRunnable is not null, then add to the message queue
          if (mPendingRunnable != null) {
              mHandler.post(mPendingRunnable);
          }
  
          // show or hide the fab button
          toggleFab();
  
          //Closing drawer on item click
          drawer.closeDrawers();
  
          // refresh toolbar menu
          invalidateOptionsMenu();
      }
  
      private Fragment getHomeFragment() {
          switch (navItemIndex) {
              case 0:
                  // home
                  DashboardFragment homeFragment = new DashboardFragment();
                  return homeFragment;
              case 1:
                  // photos
                  PhotosFragment photosFragment = new PhotosFragment();
                  return photosFragment;
              case 2:
                  // movies fragment
                  DashboardFragment moviesFragment = new DashboardFragment();
                  return moviesFragment;
              case 3:
                  // notifications fragment
                  DashboardFragment notificationsFragment = new DashboardFragment();
                  return notificationsFragment;
  
              case 4:
                  // settings fragment
                  DashboardFragment settingsFragment = new DashboardFragment();
                  return settingsFragment;
              default:
                  return new DashboardFragment();
          }
      }
  
      private void setToolbarTitle() {
          getSupportActionBar().setTitle(activityTitles[navItemIndex]);
      }
  
      private void selectNavMenu() {
          navigationView.getMenu().getItem(navItemIndex).setChecked(true);
      }
  
      private void setUpNavigationView() {
          //Setting Navigation View Item Selected Listener to handle the item click of the navigation menu
          navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
  
              // This method will trigger on item Click of navigation menu
              @Override
              public boolean onNavigationItemSelected(MenuItem menuItem) {
  
                  //Check to see which item was being clicked and perform appropriate action
                  switch (menuItem.getItemId()) {
                      //Replacing the main content with ContentFragment Which is our Inbox View;
                      case R.id.nav_home:
                          navItemIndex = 0;
                          CURRENT_TAG = TAG_HOME;
                          break;
                      case R.id.nav_photos:
                          navItemIndex = 1;
                          CURRENT_TAG = TAG_PHOTOS;
                          break;
                      case R.id.nav_movies:
                          navItemIndex = 2;
                          CURRENT_TAG = TAG_MOVIES;
                          break;
                      case R.id.nav_notifications:
                          navItemIndex = 3;
                          CURRENT_TAG = TAG_NOTIFICATIONS;
                          break;
                      case R.id.nav_settings:
                          navItemIndex = 4;
                          CURRENT_TAG = TAG_SETTINGS;
                          break;
                      case R.id.nav_about_us:
                          // launch new intent instead of loading fragment
                          startActivity(new Intent(HomeActivity.this, MainActivity.class));
                          drawer.closeDrawers();
                          return true;
                      case R.id.nav_privacy_policy:
                          // launch new intent instead of loading fragment
                          startActivity(new Intent(HomeActivity.this, MainActivity.class));
                          drawer.closeDrawers();
                          return true;
                      default:
                          navItemIndex = 0;
                  }
  
                  //Checking if the item is in checked state or not, if not make it in checked state
                  if (menuItem.isChecked()) {
                      menuItem.setChecked(false);
                  } else {
                      menuItem.setChecked(true);
                  }
                  menuItem.setChecked(true);
  
                  loadHomeFragment();
  
                  return true;
              }
          });
  
  
          ActionBarDrawerToggle actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.openDrawer, R.string.closeDrawer) {
  
              @Override
              public void onDrawerClosed(View drawerView) {
                  // Code here will be triggered once the drawer closes as we dont want anything to happen so we leave this blank
                  super.onDrawerClosed(drawerView);
              }
  
              @Override
              public void onDrawerOpened(View drawerView) {
                  // Code here will be triggered once the drawer open as we dont want anything to happen so we leave this blank
                  super.onDrawerOpened(drawerView);
              }
          };
  
          //Setting the actionbarToggle to drawer layout
          drawer.setDrawerListener(actionBarDrawerToggle);
  
          //calling sync state is necessary or else your hamburger icon wont show up
          actionBarDrawerToggle.syncState();
      }
  
      @Override
      public void onBackPressed() {
          if (drawer.isDrawerOpen(GravityCompat.START)) {
              drawer.closeDrawers();
              return;
          }
  
          // This code loads home fragment when back key is pressed
          // when user is in other fragment than home
          if (shouldLoadHomeFragOnBackPress) {
              // checking if user is on other navigation menu
              // rather than home
              if (navItemIndex != 0) {
                  navItemIndex = 0;
                  CURRENT_TAG = TAG_HOME;
                  loadHomeFragment();
                  return;
              }
          }
  
          super.onBackPressed();
      }
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
          // Inflate the menu; this adds items to the action bar if it is present.
  
          // show menu only when home fragment is selected
          if (navItemIndex == 0) {
              getMenuInflater().inflate(R.menu.home, menu);
          }
  
          // when fragment is notifications, load the menu created for notifications
          if (navItemIndex == 3) {
              getMenuInflater().inflate(R.menu.notifications, menu);
          }
          return true;
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          // Handle action bar item clicks here. The action bar will
          // automatically handle clicks on the Home/Up button, so long
          // as you specify a parent activity in AndroidManifest.xml.
          int id = item.getItemId();
  
          //noinspection SimplifiableIfStatement
          if (id == R.id.action_logout) {
              Toast.makeText(getApplicationContext(), "Logout user!", Toast.LENGTH_LONG).show();
              return true;
          }
  
          // user is in notifications fragment
          // and selected 'Mark all as Read'
          if (id == R.id.action_mark_all_read) {
              Toast.makeText(getApplicationContext(), "All notifications marked as read!", Toast.LENGTH_LONG).show();
          }
  
          // user is in notifications fragment
          // and selected 'Clear All'
          if (id == R.id.action_clear_notifications) {
              Toast.makeText(getApplicationContext(), "Clear all notifications!", Toast.LENGTH_LONG).show();
          }
  
          return super.onOptionsItemSelected(item);
      }
  
      // show or hide the fab
      private void toggleFab() {
          if (navItemIndex == 0)
              fab.show();
          else
              fab.hide();
      }
  }
  </code></pre>
  
  <p>Other activities and fragments are irrelevant in my opinion. So I'll give you the layout files. Here is the drawer:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:id="@+id/drawer_layout"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:fitsSystemWindows="true"
      tools:openDrawer="start"&gt;
  
      &lt;include
          layout="@layout/app_bar_home"
          android:layout_width="match_parent"
          android:layout_height="match_parent" /&gt;
  
      &lt;android.support.design.widget.NavigationView
          android:id="@+id/nav_view"
          android:layout_width="wrap_content"
          android:layout_height="match_parent"
          android:layout_gravity="start"
          android:fitsSystemWindows="true"
          app:headerLayout="@layout/nav_header_home"
          app:menu="@menu/activity_home_drawer" /&gt;
  
  &lt;/android.support.v4.widget.DrawerLayout&gt;
  </code></pre>
  
  <p>The header of the drawer itself:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      android:id="@+id/view_container"
      android:layout_width="match_parent"
      android:layout_height="@dimen/nav_header_height"
      android:gravity="bottom"
      android:orientation="vertical"
      android:theme="@style/ThemeOverlay.AppCompat.Dark"&gt;
  
      &lt;ImageView
          android:id="@+id/img_header_bg"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:scaleType="fitXY"
          android:src="@drawable/favicon" /&gt;
  
      &lt;LinearLayout
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_centerVertical="true"
          android:orientation="vertical"
          android:padding="@dimen/activity_horizontal_margin"&gt;
  
          &lt;ImageView
              android:id="@+id/img_profile"
              android:layout_width="@dimen/profile_width"
              android:layout_height="@dimen/profile_height"
              android:paddingTop="@dimen/nav_header_vertical_spacing"
              app:srcCompat="@android:drawable/sym_def_app_icon" /&gt;
  
          &lt;TextView
              android:id="@+id/name"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:paddingTop="@dimen/nav_header_vertical_spacing"
              android:textAppearance="@style/TextAppearance.AppCompat.Body1" /&gt;
  
          &lt;TextView
              android:id="@+id/website"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content" /&gt;
  
      &lt;/LinearLayout&gt;
  &lt;/RelativeLayout&gt;
  </code></pre>
  
  <p>Please help, I'm really lost now.</p>