Bug 1138529 - Add null checks mDoorHangerPopup access in toolbar editing state. r=liuche

Note: the call in onStartEditing is intended to fix the bug, but the call in
onStopEditing is similar enough that it makes sense to guard it as well.

--HG--
extra : rebase_source : 44dbeebd1ea6ed31f8437decb52c75457f27eb7c

mobile/android/base/BrowserApp.java

@@ -1027,7 +1027,9 @@ public class BrowserApp extends GeckoApp
                 }
 
                 // Temporarily disable doorhanger notifications.
-                mDoorHangerPopup.disable();
+                if (mDoorHangerPopup != null) {
+                    mDoorHangerPopup.disable();
+                }
             }
         });
 
@@ -1050,7 +1052,9 @@ public class BrowserApp extends GeckoApp
                 hideHomePager();
 
                 // Re-enable doorhanger notifications. They may trigger on the selected tab above.
-                mDoorHangerPopup.enable();
+                if (mDoorHangerPopup != null) {
+                    mDoorHangerPopup.enable();
+                }
             }
         });