Fix NPE if user finishes activity quickly.

b/17646987

Change-Id: I6c3aa94f8b557449af410b323a6bbdfaa389015e

samples/SupportLeanbackDemos/src/com/example/android/leanback/BrowseErrorActivity.java

@@ -50,6 +50,9 @@ public class BrowseErrorActivity extends Activity
         handler.postDelayed(new Runnable() {
             @Override
             public void run() {
+                if (getFragmentManager().isDestroyed()) {
+                    return;
+                }
                 getFragmentManager().beginTransaction().remove(mSpinnerFragment).commit();
                 mErrorFragment.setErrorContent(getResources());
             }