This appears to happen with any keyboard key on the final dialog.
If I may hazard a guess, I'd say it's something like the event handler for keyboard keys remaining active while the window has already closed(which happens between the confirmation dialog and the final dialog, which would be why it only happens with the latter), thus pressing a keyboard key leads to it attempting to access disposed resources.
System.NullReferenceException: Object reference not set to an instance of an object.
at VoiceAttack.frmKeyPress.(UInt16 )
at VoiceAttack.frmKeyPress.(Object , KeyEventArgs )
at ..( )
at ..(Int32 , IntPtr )
at ..(Int32 , Int32 , IntPtr )