Fix BadWindow error when stopping embedded game on Linux

[Hilderin]

• Fixes "Unhandled XServer error" is printed every time the project is stopped when running from the editor with game embedding #102039

The error was introduced in the PR #101895 on Linux when using the Stop button when the game is embedded. When using the Stop button the game process is killed and the window id becomes invalid. Even with the XSetErrorHandler(&bad_window_error_handler), the error is thrown on the next x11 call. Adding a little check to see if the window is still valid before executing the XSendEvent fixed the issue.

The error was:

INTERNAL ERROR: Unhandled XServer error: BadWindow (invalid Window parameter)
   Major opcode of failed request: 25
   Serial number of failed request: 0
   Current serial number in output stream: 8195764

CC @YeldhamDev

[YeldhamDev]

Fixes it on my end, good job!

[bruvzg]

Issue is reproducable on XWayland and is fixed.

[Repiteo]

Thanks!