The problem is in source
So, you’ve got to change it to a proper one
So do that, edit the source of the fxml file here
AnchorPane root = (AnchorPane) FXMLLoader.load(Main.class.getResource("LoginGUI.fxml"));
with this one
AnchorPane root = (AnchorPane) FXMLLoader.load(Main.class.getResource("/packagename