Fix crash when event listener not created

visualizer.c

@@ -458,8 +458,10 @@ void visualizer_init(void) {
 void update_status(bool changed) {
     if (changed) {
         GSourceListener* listener = geventGetSourceListener(layer_changed_event, NULL);
+        if (listener) {
             geventSendEvent(listener);
         }
+    }
 #ifdef USE_SERIAL_LINK
     static systime_t last_update = 0;
     systime_t current_update = chVTGetSystemTimeX();