title: Application fails to start once onClickListener is set
body: <p>Being new to android development I'm expecting few hiccups there and now, but this one got me scratching my head.</p>
  
  <p>I started out by creating few buttons in fragment_main.xml file (I'm working in Android Studio and using Genymotion as my Emulator)</p>
  
  <p>The reason why I mentioned this is because I use activity_main.xml as a view in my MainActivity java file (I'm still a little bit confused with these two files, but from what I've heard activity_main.xml uses layout in fragment_main.xml file?)</p>
  
  <p>Anyways, Application runs as expected, but gives "Unfortunately [Application Name] has stopped" error once I add onClick Listener to <code>getAboutButton</code></p>
  
  <p>My MainActivity Looks like this at the moment (including imports as well just in case):</p>
  
  <pre><code>package com.example.braintraininggame.app;
  
  import android.os.Bundle;
  import android.support.v4.app.Fragment;
  import android.support.v7.app.ActionBarActivity;
  import android.view.LayoutInflater;
  import android.view.Menu;
  import android.view.View;
  import android.view.ViewGroup;
  import android.widget.Button;
  
  public class MainActivity extends ActionBarActivity {
  
      /*
          Declare variables.
      */
      Button getAboutButton;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          if (savedInstanceState == null) {
              getSupportFragmentManager().beginTransaction()
                      .add(R.id.container, new PlaceholderFragment())
                      .commit();
          }
  
          /*
              Assign views to variables.
           */
          getAboutButton = (Button) findViewById(R.id.buttonAbout);
  
          /*
              Create Listeners.
           */
          getAboutButton.setOnClickListener(new View.OnClickListener() {
              public void onClick(View v) {
  
              }
          });
  
  
      }
  
      /*
          Methods
       */
  
  
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
  
          // Inflate the menu; this adds items to the action bar if it is present.
          getMenuInflater().inflate(R.menu.main, menu);
          return true;
      }
  
      /**
       * A placeholder fragment containing a simple view.
       */
      public static class PlaceholderFragment extends Fragment {
  
          public PlaceholderFragment() {
          }
  
          @Override
          public View onCreateView(LayoutInflater inflater, ViewGroup container,
                  Bundle savedInstanceState) {
              View rootView = inflater.inflate(R.layout.fragment_main, container, false);
              return rootView;
          }
      }
  
  }
  </code></pre>
  
  <p>I assume this is stacktrace:</p>
  
  <pre><code>java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.braintraininggame.app/com.example.braintraininggame.app.MainActivity}: java.lang.NullPointerException
              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
              at android.app.ActivityThread.access$600(ActivityThread.java:141)
              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
              at android.os.Handler.dispatchMessage(Handler.java:99)
              at android.os.Looper.loop(Looper.java:137)
              at android.app.ActivityThread.main(ActivityThread.java:5041)
              at java.lang.reflect.Method.invokeNative(Native Method)
              at java.lang.reflect.Method.invoke(Method.java:511)
              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
              at dalvik.system.NativeStart.main(Native Method)
  </code></pre>
  
  <p>Android studio says that it is caused by line 38, in this case line that follows right after "Create Listener" comment.</p>