Unhandled exception when opening then closing and once again opening a new form

Question

I'm getting ObjectDisposedException was unhandled with message Cannot access a disposed object.

This happens when initialize this child form at the beginning of my class by MyForm myForm = new MyForm(); and then adding some text to my text box of that form by myForm.txtBox.AppendText("Text"); and then opening my new form my form by using some button with code myForm.Show();. Now when my job is done I can close the form. Now, when I want to display the data again I'm getting that exception.

I want to keep the content of text box in my new form, but it seems like there is a problem that I haven't disposed everything in it.

How to avoid this so I can view the new forms content any time I press button?