I also got the NullException. It seems that Quiz calls its Update method before the Timer calls its Update. This means that loadNextQuestion is false so the currentQustion has not been set. I “fixed” it by doing:
else if(!hasAnsweredEarly && !timer.isAnsweringQuestion && currentQuestion != null)
in the Quiz Update method.
I even commented out all my Timer and Quiz classes and used the ones from Gitlab and they gave the same error.
Update: I completed the whole course and it turns out this bug is fixed in the GameManager video. However this bug fix should probably be moved to the Lists video as whilst this bug is in place the game execution halts on the exception.