Bug 1366352 - Post: check anchor is set before trying to show GeckoPopupMenu r=nechen

This means we'll at least see a sensible exception, instead of an NPE when
no anchor is set. That makes it easier to debug cases where no anchor was
set (see e.g. the main commit from this bug).

MozReview-Commit-ID: CzsZaHvnZ6y

--HG--
extra : rebase_source : 2aaf737b49367d9e70fc62a1226e038c79f573e3

mobile/android/base/java/org/mozilla/gecko/widget/GeckoPopupMenu.java

@@ -137,6 +137,10 @@ public class GeckoPopupMenu implements GeckoMenu.Callback,
      * Show the inflated menu.
      */
     public void show() {
+        if (mAnchor == null) {
+            throw new IllegalStateException("GeckoPopupMenu.show() called without preceeding call to setAnchor()");
+        }
+
         if (!mMenuPopup.isShowing())
             mMenuPopup.showAsDropDown(mAnchor);
     }