Currently when you are retrying the last question, there is only one action: the retry, which is then automatically executed and gives a strange flickery effect without the student really knowing what happened. This PR adds a stop action which stops the exam by not setting a next question. This implies that there are 2 possible actions in the end, which prevents the flickery effect by letting the student decide whether to retry or not. It also fixes the next_diagnostic_question which didn't check for null.
Currently when you are retrying the last question, there is only one action: the retry, which is then automatically executed and gives a strange flickery effect without the student really knowing what happened. This PR adds a stop action which stops the exam by not setting a next question. This implies that there are 2 possible actions in the end, which prevents the flickery effect by letting the student decide whether to retry or not. It also fixes the next_diagnostic_question which didn't check for null.