Fix: freed building error

scripts/build/build_controller.gd

@@ -49,9 +49,10 @@ func is_mouse_tile_supported() -> bool:
 	if below_tile_data != null:
 		return true
 	
-	var building: Building = structures.get(below_tile_pos)
-	if is_instance_valid(building) and not building is Plant:
-		return true
+	if is_instance_valid(structures.get(below_tile_pos)):
+		var building: Building = structures.get(below_tile_pos)
+		if not building is Plant:
+			return true
 	
 	return false