Xcode Swift 3.0 user Authentication : ok, guys, I know this is silly but I'm having a bit of a trouble shooting and need help. I created a log out button to log users out of the account but when touching the log out button I get an error showing this 

`libc++abi.dylib: terminating with uncaught exception of type NSException
`  

[my Error][1]


[Logout btoon][2]


  [1]: https://i.stack.imgur.com/rkCFU.png
  [2]: https://i.stack.imgur.com/XbAgp.png

Is there a way to fix this problem? Thanks to anyone helping.