Bug 774156 - Fix NPE in Tabs when "Don't keep activities" is enabled. r=mfinkle

--HG--
extra : rebase_source : 349b50529c10e34197975373233ccc5525ade7fe

mobile/android/base/Tabs.java

@@ -101,7 +101,8 @@ public class Tabs implements GeckoEventListener {
         mSelectedTab = tab;
         GeckoApp.mAppContext.mMainHandler.post(new Runnable() { 
             public void run() {
-                GeckoApp.mAppContext.mFormAssistPopup.hide();
+                if (GeckoApp.mAppContext.mFormAssistPopup != null)
+                    GeckoApp.mAppContext.mFormAssistPopup.hide();
                 if (isSelectedTab(tab)) {
                     String url = tab.getURL();
                     notifyListeners(tab, TabEvents.SELECTED);