JavaFX: Exception in thread "JavaFX Application Thread" java.lang.IllegalStateException: Location is not set

Question

I am working on Java FX project I'm created FXML mainScreen.fxml file and tried to load it using FXML loader but it always says "Location is not set" Here is my entry point of my project that is a Java file Error description and some files

This is class MainScreen

package history.media;

import java.io.IOException;
import java.util.List;

import javax.swing.JFrame;

import history.entity.*;
import history.entity.Character;
import javafx.application.Platform;
import javafx.embed.swing.JFXPanel;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import store.Store;

public class MainScreen extends JFrame {
    private Store store;
    public MainScreen(Store store) {
        super();
        this.store = store;
        JFXPanel fxPanel = new JFXPanel();
        this.add(fxPanel);
        this.setTitle("main");
        setVisible(true);
        JFrame frame = this;
        Platform.runLater(new Runnable() {
            
            @Override
            public void run() {
                try {
                    FXMLLoader loader = new FXMLLoader(getClass().getResource("/mainScreen.fxml"));
                    MainScreenController controller = new MainScreenController(store, frame);
                    loader.setController(controller);

                    Parent root = loader.load();
                    fxPanel.setScene(new Scene(root, 1024, 768));
                    fxPanel.setSize(1024, 768);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
    }