Why the follow code when run in Eclipse, frame object will be null (null pointer exception)

But when I run in Processing IDE, the frame object is not null?

public class MyClass extends PApplet {
   public void init() {
       frame.setUndecorated(true);
       super.init();
   }
}

in eclipse I normaly pass the frame object to the PApplet when creating it. so you can access it from there as in the P5 IDE.

public class MainClass extends Frame {

 (...)

 public MainClass() {
   (...)
    Applet p5sketch = new P5Sketch(this);
   (...)
 }

 public static void main(String[] s) {
   new Main().setVisible(true);
 }

}

public class P5Sketch extends PApplet {

 Frame frame;
 public P5Sketch(Frame frame) {
   this.frame = frame; // here you go
 }
}

But when I add another Class - MyMainClass.java and run it as java application. I get nothing show up in my application, anything wrong with the following code?

MyMainClass.java
==================================
public class MyMainClass extends Frame {
   public MyMainClass() {
      Applet p5sketch = new MyApplet(this);
   }
         
   public static void main(String[] s) {
      new MyMainClass().setVisible(true);
 }
}

MyApplet.java
==================================
public class MyApplet extends PApplet {
      Frame frame;
      public MyApplet(Frame frame) {
            this.frame = frame; // here you go
      }
      public void init() {
            frame.setUndecorated(true);
            super.init();
      }
      public void setup() {
            // do some setup
      }
}