Bug 1325021 - Set mDoorHangerPopup to null after destroying it. r=sebastian

mobile/android/base/java/org/mozilla/gecko/BrowserApp.java

@@ -1540,8 +1540,10 @@ public class BrowserApp extends GeckoApp
 
         deleteTempFiles(getApplicationContext());
 
-        if (mDoorHangerPopup != null)
+        if (mDoorHangerPopup != null) {
             mDoorHangerPopup.destroy();
+            mDoorHangerPopup = null;
+        }
         if (mFormAssistPopup != null)
             mFormAssistPopup.destroy();
         if (mTextSelection != null)