Bug 820022 - Add null check for layerView. r=margaret

mobile/android/base/GeckoApp.java

@@ -2335,7 +2335,7 @@ abstract public class GeckoApp
             return;
         }
 
-        if (mLayerView.isFullScreen()) {
+        if (mLayerView != null && mLayerView.isFullScreen()) {
             GeckoAppShell.sendEventToGecko(GeckoEvent.createBroadcastEvent("FullScreen:Exit", null));
             return;
         }