Forget the current mode when it is finished

src/outputs.c

@@ -362,6 +362,9 @@ static void mode_handle_preferred(void *data,
 static void mode_handle_finished(void *data,
     struct zwlr_output_mode_v1 *wlr_mode) {
   struct wd_mode *mode = data;
+  if (mode->head->mode == mode) {
+    mode->head->mode = NULL;
+  }
   wl_list_remove(&mode->link);
   wd_mode_destroy(mode);
 }