Challenge: Add Menu Item Validation

I really struggled with the first challenge, but the second challenge was straightforward.
I added the following to MainWindowControler.swift

override func validateMenuItem(menuItem: NSMenuItem) -> Bool {
    if window!.firstResponder is DieView {
        return true
    else {
        return false

Then added a dummy text-field to the MainWindowController.xib so I could take window.FirstResponder away from the dice (just for test purposes).