Laravel: CSRF Token invalid after user logs out?

A logged in user can edit his profile. If he presses "Save" then the data will be send by AJAX and he gets an success message:

If the user logs out in a different tab, then the "Save" button does not work anymore. I would like to let the user know with an ajax warning(just as above, but in red instead of green), that he is logged out.

This is my code:

public function __construct()
{
  $this->middleware('auth',['except' => 'updateContact']);
}

public function updateContact(Request $request)
{
  if(!Auth::check()){
   return json_encode(['status' => false, 'msg' => 'You are not logged in! Chanes are not saved!']);
}

However, this does not work. THe method is not returning anything. Instead I get a warning

The page has expired due to inactivity.

It looks to me that the CSRF token is invalid as soon as I logout a user. Why is that and how can I fix it?

via Chebli Mohamed