My ' X ' Button does not work in the client!

The ' X ' button does not work in the main client. It has occurred two days in a row, and I'm not sure why. I can press the ' X ' repeatedly but nothing happens. The minimize button works fine though. In order to exit the League client I have to right-click the icon on my task bar and close from there.

Has anyone else experienced this issue? I know it's not some game breaking bug, but it is annoying.