Merge backout of 777fbc99f40b and 27e4b9cbed74 due to leak bustage

mobile/android/base/Tabs.java

@@ -101,6 +101,7 @@ public class Tabs implements GeckoEventListener {
         mSelectedTab = tab;
         GeckoApp.mAppContext.mMainHandler.post(new Runnable() { 
             public void run() {
+                if (GeckoApp.mAppContext.mFormAssistPopup != null)
                     GeckoApp.mAppContext.mFormAssistPopup.hide();
                 if (isSelectedTab(tab)) {
                     String url = tab.getURL();