When you deploy your application, you typically create a jar file which includes the class files and all the resources. You need to load the fxml file from inside the jar file, i.e. as a resource. In this context, the path element ".." doesn't really mean anything.
Use
public static final String
DEFAULT_SCREEN_FXML = "/client/stuff/fxml/Default.fxml";
(the path is relative to the classpath).
and of course load it with
FXMLLoader loader = new FXMLLoader(getClass().getResource(DEFAULT_SCREEN_FXML));
or something similar.