title: error onClickListener MainActivity Android
body: <p>I hope you can help me with this error.</p>
  
  <p>I want to associate an activity to a button but I have this error in the logcat (updated).</p>
  
  <pre><code>02-17 18:04:27.033: D/AndroidRuntime(1850): Shutting down VM
  02-17 18:04:27.034: E/AndroidRuntime(1850): FATAL EXCEPTION: main
  02-17 18:04:27.034: E/AndroidRuntime(1850): Process: com.cm.georganizer, PID: 1850
  02-17 18:04:27.034: E/AndroidRuntime(1850): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cm.georganizer/com.cm.georganizer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.ActivityThread.access$800(ActivityThread.java:144)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.os.Handler.dispatchMessage(Handler.java:102)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.os.Looper.loop(Looper.java:135)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.ActivityThread.main(ActivityThread.java:5221)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at java.lang.reflect.Method.invoke(Native Method)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at java.lang.reflect.Method.invoke(Method.java:372)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
  02-17 18:04:27.034: E/AndroidRuntime(1850): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at com.cm.georganizer.MainActivity.onCreate(MainActivity.java:47)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.Activity.performCreate(Activity.java:5933)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
  02-17 18:04:27.034: E/AndroidRuntime(1850):     ... 10 more
  02-17 18:09:27.363: I/Process(1850): Sending signal. PID: 1850 SIG: 9
  </code></pre>
  
  <p>This is my MainActivity (updated)</p>
  
  <pre><code>import android.content.Intent;
  import android.os.Bundle;
  import android.support.v4.app.Fragment;
  import android.support.v4.app.FragmentManager;
  import android.util.SparseIntArray;
  import android.view.Menu;
  import android.view.View;
  import android.widget.Toast;
  
  import android.database.Cursor;
  import android.support.v7.app.ActionBarActivity;
  import android.view.View.OnClickListener;
  import android.widget.AdapterView;
  import android.widget.AdapterView.OnItemClickListener;
  import android.widget.Button;
  import android.widget.ListView;
  import android.widget.SimpleCursorAdapter;
  import android.widget.TextView;
  
  import java.util.ArrayList;
  import java.util.List;
  
  import br.liveo.interfaces.NavigationLiveoListener;
  import br.liveo.navigationliveo.NavigationLiveo;
  
  
  public class MainActivity extends NavigationLiveo implements NavigationLiveoListener {
  
  
      Button btnAgregarMiembro;
      ListView lista;
      SQLControlador dbconeccion;
      TextView tv_miemID, tv_miemNombre;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.fragment_main);
  
          dbconeccion = new SQLControlador(this);
          dbconeccion.abrirBaseDeDatos();
          btnAgregarMiembro = (Button) findViewById(R.id.btnAgregarMiembro);
          lista = (ListView) findViewById(R.id.listViewMiembros);
  
          //acción del boton agregar miembro
          btnAgregarMiembro.setOnClickListener(new OnClickListener() {
              @Override
              public void onClick(View v) {
                  Intent iagregar = new Intent(MainActivity.this, AgregarMiembro.class);
                  startActivity(iagregar);
              }
          });
  
          // Tomar los datos desde la base de datos para poner en el curso y después en el adapter
          Cursor cursor = dbconeccion.leerDatos();
  
          String[] from = new String[] {
                  DBhelper.MIEMBRO_ID,
                  DBhelper.MIEMBRO_NOMBRE
          };
          int[] to = new int[] {
                  R.id.miembro_id,
                  R.id.miembro_nombre
          };
  
          SimpleCursorAdapter adapter = new SimpleCursorAdapter(
                  MainActivity.this, R.layout.formato_fila, cursor, from, to);
  
          adapter.notifyDataSetChanged();
          lista.setAdapter(adapter);
  
          // acción cuando hacemos click en item para poder modificarlo o eliminarlo
          lista.setOnItemClickListener(new OnItemClickListener() {
              @Override
              public void onItemClick(AdapterView parent, View view, int position, long id) {
  
                  tv_miemID = (TextView) view.findViewById(R.id.miembro_id);
                  tv_miemNombre = (TextView) view.findViewById(R.id.miembro_nombre);
  
                  String aux_miembroId = tv_miemID.getText().toString();
                  String aux_miembroNombre = tv_miemNombre.getText().toString();
  
                  Intent modify_intent = new Intent(getApplicationContext(), ModificarMiembro.class);
                  modify_intent.putExtra("miembroId", aux_miembroId);
                  modify_intent.putExtra("miembroNombre", aux_miembroNombre);
                  startActivity(modify_intent);
              }
          });
      }  //termina el onCreate
  
  
  
      public List&lt;String&gt; mListNameItem;
  
      @Override
      public void onUserInformation() {
          //User information here
          //this.mUserName.setText("User");
          //this.mUserEmail.setText("rudsonlive@gmail.com");
          //this.mUserPhoto.setImageResource(R.drawable.ic_rudsonlive);
          this.mUserBackground.setImageResource(R.drawable.ic_app_background);
      }
  
      @Override
      public void onInt(Bundle savedInstanceState) {
          //Creation of the list items is here
  
          // set listener {required}
          this.setNavigationListener(this);
  
          //First item of the position selected from the list
          this.setDefaultStartPositionNavigation(0);
  
          // name of the list items
          mListNameItem = new ArrayList&lt;&gt;();
          mListNameItem.add(0, getString(R.string.index));
          mListNameItem.add(1, getString(R.string.add_event));
  
          // icons list items
          List&lt;Integer&gt; mListIconItem = new ArrayList&lt;&gt;();
          mListIconItem.add(0, R.drawable.ic_inbox_black_24dp);
          mListIconItem.add(1, R.drawable.ic_star_black_24dp); //Item no icon set 0
  
          //{optional} - Among the names there is some subheader, you must indicate it here
          List&lt;Integer&gt; mListHeaderItem = new ArrayList&lt;&gt;();
          mListHeaderItem.add(4);
  
          //{optional} - Among the names there is any item counter, you must indicate it (position) and the value here
          SparseIntArray mSparseCounterItem = new SparseIntArray(); //indicate all items that have a counter
          mSparseCounterItem.put(0, 0);
          mSparseCounterItem.put(1, 0);
  
          //If not please use the FooterDrawer use the setFooterVisible(boolean visible) method with value false
          //this.setFooterInformationDrawer(R.string.settings, R.drawable.ic_settings_black_24dp);
  
          this.setNavigationAdapter(mListNameItem, mListIconItem, mListHeaderItem, mSparseCounterItem);
      }
  
      @Override
      public void onItemClickNavigation(int position, int layoutContainerId) {
  
          FragmentManager mFragmentManager = getSupportFragmentManager();
  
          Fragment mFragment = new FragmentMain().newInstance(mListNameItem.get(position));
  
          switch (position) {
              case 0:
                  mFragment = new FragmentMain().newInstance(mListNameItem.get(position));
                  break;
  
              case 1:
                  mFragment = new FragmentEvent().newInstance(mListNameItem.get(position));
                  break;
          }
  
          if (mFragment != null){
              mFragmentManager.beginTransaction().replace(layoutContainerId, mFragment).commit();
          }
      }
  
      @Override
      public void onPrepareOptionsMenuNavigation(Menu menu, int position, boolean visible) {
  
          //hide the menu when the navigation is opens
          switch (position) {
              case 0:
                  menu.findItem(R.id.menu_add).setVisible(!visible);
                  menu.findItem(R.id.menu_search).setVisible(!visible);
                  break;
  
              case 1:
                  menu.findItem(R.id.menu_add).setVisible(!visible);
                  menu.findItem(R.id.menu_search).setVisible(!visible);
                  break;
          }
      }
  
      @Override
      public void onClickUserPhotoNavigation(View v) {
          //user photo onClick
          Toast.makeText(this, R.string.open_user_profile, Toast.LENGTH_SHORT).show();
      }
  
      @Override
      public void onClickFooterItemNavigation(View v) {
          //footer onClick
          startActivity(new Intent(this, SettingsActivity.class));
      }
  }
  </code></pre>
  
  <p>fragment_main.xml</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"&gt;
  
      &lt;ImageView
          android:id="@+id/imgLiveo"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_alignParentTop="true"
          android:layout_centerHorizontal="true"
          android:contentDescription="@string/clear"
          android:src="@drawable/logo" /&gt;
  
      &lt;TextView
          android:id="@+id/txtTitle"
          android:layout_width="fill_parent"
          android:layout_height="wrap_content"
          android:layout_below="@+id/imgLiveo"
          android:gravity="center"
          android:layout_marginTop="16dp"
          android:text="@string/clear"
          android:textAppearance="?android:attr/textAppearanceMedium" /&gt;
  
      &lt;Button
          android:id="@+id/btnAgregarMiembro"
          android:layout_width="fill_parent"
          android:layout_below="@+id/txtTitle"
          android:layout_height="wrap_content"
          android:text="Agregar miembro" /&gt;
  
      &lt;ListView
          android:id="@+id/listViewMiembros"
          android:layout_width="match_parent"
          android:layout_below="@+id/btnAgregarMiembro"
          android:layout_height="wrap_content"
          android:dividerHeight="2dp" &gt;
      &lt;/ListView&gt;
  
  &lt;/RelativeLayout&gt;
  </code></pre>
  
  <p>I have made several attempts to fix it but do not work</p>
  
  <p>Beforehand thank you very much for your help</p>