Empty Dropdown list for properties of a custom Java Bean in Eclipse WindowBuilder

I am using Eclipse 2020-06 (4.16.0) with WindowBuilder 1.9.4.202002151729. I am trying to create my own Java Bean but I am facing a problem when I try to change a custom String type property on WindowBuilder Design tab. The problem is that property is not editable, three dots next to it do not appear as they do in properties of swing components. Instead, an arrow is displayed and an empty dropdown list appears when I click on it:

I have been trying many different things but I am not able to edit my properties. Java Compiler compliance level is set to 1.8 and the code of my Bean is as follows:

public class TestingBean extends JLabel
{
    private String myString;
    
    public TestingBean()
    {
        this.myString = "";
    }
    
    public String getMyString()
    {
        return myString;
    }

    public void setMyString(String myString)
    {
        this.myString = myString;
        this.setText("My text is: " + myString);
    }
}

Also, I was looking for this problem and I have not found any information. Anybody knows if there is any known bug like this one or if I could be doing something wrong?

Thank you in advance.

Replacing OpenJDK by Oracle JDK in Project > Properties > Java Build Path > Libraries and rebuilding the Java Bean class fixed this problem for me.