There are really only a few situations in which the script in Learning Aid could be executed in a way that would cause the game to crash.
- If ClassTrainerFrame_Update fires before the trainer list is populated
- If ClassTrainerFrame_Update fires while you aren't talking to a trainer
- If GetNumTrainerServices is returning an incorrect number of items
- Another addon is interfering with how this one operates
I still think that it's Learning Aid that's causing the crash, but I absolutely cannot reproduce the combination of events that trigger it.
Talking to a trainer at some point beforehand should be required for it to break, since that's what populates GetNumTrainerServices. This would explain why reloading prevents the crash because the function would go back to returning 0.
I want to say another addon is causing the TRAINER_UPDATE event which is causing ClassTrainerFrame_Update to be called resulting in a crash, but so far the only function I've been able to trigger TRAINER_UPDATE with is
SetTrainerServiceTypeFilter and GetNumTrainerServices is being reset to 0 when I call it.