title: NullPointerException in MapView
body: <p>When I push back button <code>ListView</code> activity to return to <code>MapViewActivity</code>, I have got the error below.</p>
  
  <p>I don't really know what is wrong since it isn't showing where the error is in my code.</p>
  
  <p>Any suggestion would be appreciated!</p>
  
  <pre><code>07-20 15:49:51.558: E/AndroidRuntime(2794): FATAL EXCEPTION: main
  07-20 15:49:51.558: E/AndroidRuntime(2794): java.lang.NullPointerException
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.SurfaceView.updateWindow(SurfaceView.java:497)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.SurfaceView.updateWindow(SurfaceView.java:408)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.SurfaceView.dispatchDraw(SurfaceView.java:352)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.drawChild(ViewGroup.java:1644)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.drawChild(ViewGroup.java:1644)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.View.draw(View.java:6936)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.widget.FrameLayout.draw(FrameLayout.java:357)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.drawChild(ViewGroup.java:1646)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.drawChild(ViewGroup.java:1644)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.View.draw(View.java:6936)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.widget.FrameLayout.draw(FrameLayout.java:357)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1926)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewRoot.draw(ViewRoot.java:1527)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewRoot.performTraversals(ViewRoot.java:1263)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1865)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.os.Handler.dispatchMessage(Handler.java:99)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.os.Looper.loop(Looper.java:130)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at android.app.ActivityThread.main(ActivityThread.java:3687)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at java.lang.reflect.Method.invokeNative(Native Method)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at java.lang.reflect.Method.invoke(Method.java:507)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:847)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605)
  07-20 15:49:51.558: E/AndroidRuntime(2794):     at dalvik.system.NativeStart.main(Native Method)
  </code></pre>