fix error that happens after the GUI is freed

godot/monitor/Monitor.gd

@@ -99,8 +99,9 @@ func _ready() -> void:
 
 func _physics_process(_delta: float) -> void:
 	if monitor_enabled:
-		if not $MonitorGUI.visible:
+		if has_node("MonitorGUI") and not $MonitorGUI.visible:
 			$MonitorGUI.visible = true
+
 		if started and not get_tree().paused:
 			var frame = empty_frame.duplicate(true)
 			frame["coins"] = coins