如何正确继承 FXML 带注释的属性以使其在子类中可用？

package canbusgui;

import javafx.application.application;
import javafx.fxml.fxmlloader;
import javafx.scene.scene;
import javafx.stage.stage;

import java.io.ioexception;

public class mainapplication extends application {
    public static stage stage = null;

    @override
    public void start(stage stage) throws ioexception {
        
        stage.setoncloserequest(event -> {
           system.exit(0);
        });

        mainapplication.stage = stage;

        // create a fxmlloader to load the fxml file that defines the user interface
        fxmlloader fxmlloader = new fxmlloader(mainapplication.class.getresource("mainview.fxml"));

        scene scene = new scene(fxmlloader.load());

        stage.settitle("canbus gui");
        stage.setscene(scene);
        stage.setresizable(false);
        stage.setminheight(768);
        stage.setminwidth(1366);
        stage.show();
    }

    public static void main(string[] args) {
        launch();
    }
}

package canbusgui;

import javafx.application.platform;
import javafx.collections.fxcollections;
import javafx.fxml.fxml;
import javafx.scene.input.keycode;
import javafx.scene.input.keyevent;


public class mainviewcontroller {
  @fxml
  protected textfield in_desiredvelocity;

  @fxml
  public void initialize (){

        in_desiredvelocity.addeventfilter(keyevent.key_pressed, event -> {
                if (event.getcode() == keycode.enter) {
                    try {
                        sendmessage(new pctovcumessage("222"));
                    }
                    catch (exception e) {
                        throw new runtimeexception(e);
                    }
                }
        });
  }
  

  public void sendmessage(canmessage message) throws exception {
        
        message.constructdata();
       
        message.validateinputs();
        
        byte[] data = message.getdata();

        // send the data array to the can bus
        canbuscontroller.sendcommand(hexformat.fromhexdigits(message.getid()), data);
  }

}

package canbusgui;

import static java.lang.Integer.parseInt;

public abstract class canMessage extends MainViewController{

    // Declare common attributes
    protected String id;
    protected byte[] data;

    public canMessage(String id) {
        this.id = id;
        // Initialize an empty byte array for data
        this.data = new byte[8];
    }


    // Define an abstract method to construct the data array
    public abstract void constructData();

    // Define an abstract method to validate the inputs
    public abstract void validateInputs() throws Exception;

    // Define a getter method for the data array
    public byte[] getData() {
        return this.data;
    }

    public String getId() {
        return this.id;
    }

}


// Define a subclass for PC_to_VCUMessage message
class PcToVcuMessage extends canMessage {


    public PcToVcuMessage(String id) {
        // Call the superclass constructor
        super(id);
    }


    // Override the constructData method
    @Override
    public void constructData() {


        data[0] = (byte) 0;
        data[1] = (byte) 0;
        data[2] = (byte) 0;
        data[3] = (byte) parseInt(in_desiredVelocity.getText()); //HERE in_desiredVelocity is null and a NillPointerException is launched
        data[4] = (byte) 0;
        data[5] = (byte) 0;
        data[6] = (byte) 0;
        data[7] = (byte) 0;
    }

    public  void validateInputs() throws Exception{}


}

public class mainviewcontroller {
    // ...
    protected textfield indesiredvelocity;
    // ...
}

public abstract class canmessage extends mainviewcontroller {
    // ...
}

new pctovcumessage("222")

class pctovcumessage extends canmessage {

    private int desiredvelocity ;


    public pctovcumessage(string id, int desiredvelocity) {
        // call the superclass constructor
        super(id);
        this.desiredvelocity = desiredvelocity;
    }


    // override the constructdata method
    @override
    public void constructdata() {


        data[0] = (byte) 0;
        data[1] = (byte) 0;
        data[2] = (byte) 0;
        data[3] = (byte) desiredvelocity;
        data[4] = (byte) 0;
        data[5] = (byte) 0;
        data[6] = (byte) 0;
        data[7] = (byte) 0;
    }

    public  void validateinputs() throws exception{}


}

new PcToVcuMessage("222", Integer.parseInt(inDesiredVelocity.getText()))