//Remember, if you'r using a UI component(eg: button, window,textbox...) then, don't use directly UI component inside threading....
private void opendashboard1()
{
this.Invoke(
(MethodInvoker)delegate()
{
xf4 = new ProgressForm();
xf4.ShowDialog();
});
}
But tell me, Why you'r opening this Dialog inside a thread?