I think you have enough information here to file an issue on the openhab-core repo. See How to file an Issue if you need details.
If there is it will be in code. Though the fact that you are getting that log statement means that the exception is being caught and reported.
You’d have to ask on a Kitty forum. Though I suspect that by design when you hit the X it just terminates the connection. Otherwise there would be an unacceptable delay after hitting the X while it logs out of the terminals. And if there is a problem with the terminal, you’d have to wait for the network timeout before it could close the window.