Android : Dialog's object position Libgdx

I have a Dialog :

Dialog dialog=new Dialog("",style);
dialog.setSize(400, 500);
dialog.setPosition(Gdx.graphics.getWidth()/2-200, Gdx.graphics.getHeight()/2-300);

I added a two button in it:

 dialog.button(stopButton);
 dialog.button(goButton);

My problem is that I can not change the position of buttons, not even if the imposed manually the buttons remain in the same position.

how could I do to solve the problem?

Thank you