Accessviolationexception When I type ALT or TAB C# WINDOWS FORM

Question

Guys, I have a form where the Form (Plano_aula) calls the form (Call) , in this Form Call has a list of students and a Panel that stays with the Visible = false, in this same screen I call a class where does facial recognition in another Thread, but the problem is the following, when I select this panel Visible = true by facial recognition thread, I can press any key , but when I press ALT or TAB I get Exception

See the screens,

Call with Visible panel = false :

Making - the True

When I press ALT or TAB see