Windows Application Packaging Project create new instance everytime open a toast notification

Question

Windows Application Packaging Project create new instance everytime I open a toast notification. That's not what I want. The second time the app open (from toast notification), this exception fires

System.InvalidOperationException: 'Cannot set Visibility or call Show, ShowDialog, or WindowInteropHelper.EnsureHandle after a Window has closed.'

How to allow only one instance?