app: don't warn in gimp_action_history_action_activated()

if there is no "gimp". Happened after the action changes when invoking
the "quit" action.

(cherry picked from commit 87b1a27459)

app/widgets/gimpaction-history.c

@@ -359,7 +359,11 @@ gimp_action_history_action_activated (GimpAction *action)
   GList                 *link;
   GimpActionHistoryItem *item;
 
-  g_return_if_fail (history.gimp != NULL);
+  /* Silently return when called at the wrong time, like when the
+   * activated action was "quit" and the history is already gone.
+   */
+  if (! history.gimp)
+    return;
 
   config = GIMP_GUI_CONFIG (history.gimp->config);