window: Remove popovers on dispose() before unsetting focus.

The popovers may return keyboard grabs to previous widgets, so if
called after unsetting the focus, the window may be left with a
dangling GtkWidget that would cause crash at later dispose() calls.

gtk/gtkwindow.c

@@ -2664,13 +2664,13 @@ gtk_window_dispose (GObject *object)
   GtkWindow *window = GTK_WINDOW (object);
   GtkWindowPrivate *priv = window->priv;
 
+  g_hash_table_remove_all (priv->popovers);
+
   gtk_window_set_focus (window, NULL);
   gtk_window_set_default (window, NULL);
   unset_titlebar (window);
   remove_attach_widget (window);
 
-  g_hash_table_remove_all (priv->popovers);
-
   G_OBJECT_CLASS (gtk_window_parent_class)->dispose (object);
 }