Mantain checkbox state while the app is running in WPF

I'm working in MVVM, WPF and I have a popup; inside this popup is a listbox and inside the listbox I have a checkbox. The problem is: if I uncheck an item from the list box and click outside, popup disappears; if a I click again the checkbox is reseted at its initial value(all the items become checked).

So, how can I maintain the state of the popup set and stop its reseting while the app is running ? Can I do this through XAML ?

Thanks in advance !