closing a form

I made a program that has two forms.The second form gets invoked in the first form.I had set the visibility of the first form as false before the second form gets called.When i stop running the program only the second form stopped running.I could'nt close the first form.The next time when i run the program i am getting an exception saying that it is in read only moder.How do i stop runnig the first form and remove the exception