Fix bug 3198503 - NPE at MenuPopupHelper.dismiss

Make sure dismiss() can be called even when the popup is not currently
showing.

Change-Id: I48077d8b3bfe0df04bceeb056ed54cbd79a6e660

core/java/com/android/internal/view/menu/MenuPopupHelper.java

@@ -110,8 +110,10 @@ public class MenuPopupHelper implements AdapterView.OnItemClickListener, View.On
         if (isShowing()) {
             mPopup.dismiss();
         }
-        mTreeObserver.removeGlobalOnLayoutListener(this);
-        mTreeObserver = null;
+        if (mTreeObserver != null) {
+            mTreeObserver.removeGlobalOnLayoutListener(this);
+            mTreeObserver = null;
+        }
     }
 
     public boolean isShowing() {