FIX avivais#24 (android): Handling case where app is put in background with android back button and a push is subsequently received

Author: quilligana

src/android/ParsePushPlugin.java

@@ -188,6 +188,7 @@ public void onResume(boolean multitasking) {
    public void onDestroy() {
       gWebView = null;
     	gForeground = false;
+      gEventCallback = null;
 
     	super.onDestroy();
    }