[2.0.72] Crash discarding interrupt changes prompt after closing interrupt GUI

[Dit_05]

Editing a train interrupt can be cancelled with Esc, which brings up a confirmation prompt if there are any unsaved changes.
However, while the confirmation prompt is up, you can close the interrupt editor with E, which leaves the confirmation prompt up.
After that, pressing the red confirm button on the prompt will crash the game.

Exact reproduction steps:
1.) Start editing a train interrupt
2.) Make some kind of change, but don't save it
3.) Press Esc to have the game prompt whether you want to discard the changes
4.) Press E to close the interrupt editor but leave the Confirm dialog up
5.) Press Discard changes on the Confirm dialog to crash

Expected behavior: Game does not crash.
Observed behavior: Game crashes.