почему пишет ошибку Main method not found in class App или JavaFX application class must extend javafx.application.Application хотя все это есть

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import java.io.IOException;
public class App {

    
    public class hello extends Application {
        public static void main(String[] args) throws IOException{
            launch(args);
        }
        
        @Override
        public void start(Stage primaryStage) {
            primaryStage.setTitle("Hello World!");
            Button btn = new Button();
            btn.setText("Say 'Hello World'");
            btn.setOnAction(new EventHandler<ActionEvent>() {
    
                @Override
                public void handle(ActionEvent event) {
                    System.out.println("Hello World!");
                }
            });
            
            StackPane root = new StackPane();
            root.getChildren().add(btn);
            primaryStage.setScene(new Scene(root, 300, 250));
            primaryStage.show();
            
        }
    }
}