关于调用jface.dialog对话框的有关问题?

关于调用jface.dialog对话框的问题???
package   my1;

import   org.eclipse.jface.dialogs.Dialog;
import   org.eclipse.jface.dialogs.IDialogConstants;
import   org.eclipse.swt.graphics.Point;
import   org.eclipse.swt.widgets.Composite;
import   org.eclipse.swt.widgets.Control;
import   org.eclipse.swt.widgets.Shell;

public   class   mydlg2   extends   Dialog   {

/**
  *   Create   the   dialog
  *   @param   parentShell
  */
public   mydlg2(Shell   parentShell)   {
super(parentShell);
}

/**
  *   Create   contents   of   the   dialog
  *   @param   parent
  */
@Override
protected   Control   createDialogArea(Composite   parent)   {
Composite   container   =   (Composite)   super.createDialogArea(parent);
//
return   container;
}

/**
  *   Create   contents   of   the   button   bar
  *   @param   parent
  */
@Override
protected   void   createButtonsForButtonBar(Composite   parent)   {
createButton(parent,   IDialogConstants.OK_ID,   IDialogConstants.OK_LABEL,
true);
createButton(parent,   IDialogConstants.CANCEL_ID,
IDialogConstants.CANCEL_LABEL,   false);
}

/**
  *   Return   the   initial   size   of   the   dialog
  */
@Override
protected   Point   getInitialSize()   {
return   new   Point(500,   375);
}

}
在下面主菜单中死活不能调用成功？？？
private   JMenuItem   getXaMenuItem()   {
if   (xaMenuItem   ==   null)   {
xaMenuItem   =   new   JMenuItem();
xaMenuItem.setText( "eclipse_Jdlg ");
mydlg2   my12=new   mydlg2(null);   //此句出错！！
my12.open();


}
return   xaMenuItem;
}



------解决方案--------------------
错误信息是什么?