app: fix a segmentation fault crash.

gtk_widget_get_window() may return NULL. I had the case when opening
some menus in bottom of tool options (like the "Save|Restore Tool
Preset" menus). We must check this before doing anything with it.

(cherry picked from commit 0a28534a90)

app/widgets/gimpuimanager.c

@@ -1052,6 +1052,9 @@ gimp_ui_manager_item_key_press (GtkWidget     *widget,
         {
           GtkWidget *parent = gtk_widget_get_parent (widget);
           GdkWindow *window = gtk_widget_get_window (parent);
+
+          if (window)
+            {
               gint x, y;
 
               gdk_window_get_pointer (window, &x, &y, NULL);
@@ -1066,6 +1069,7 @@ gimp_ui_manager_item_key_press (GtkWidget     *widget,
                     menu_item = NULL;
                 }
             }
+        }
 
       /*  first, get the help page from the item...
        */