Check that the segue is correctly set up between the login Button and the LoginViewController. If you select the segue in your storyboard, it should highlight the button that it is connected to.
Also check that the LoginViewController that you have configured in your Storyboard has the custom class added. Select the ViewController in your storyboard (circled) and then in the Identity Inspector make sure that the Custom Class is populated.
If I remove that class and run the project on my simulator, this is the error I receive:
*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<UIViewController 0x7f9b9cd23e40> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key emailTextField.'