Create a quiz with How questions behave: immediate feedback. Go to the Edit quiz page. Add two true-false questions to the quiz. Go into the database, into the mdl_questions table. In the row for the second question, change the value in the qtype column to somethng non existent like 'unknown'. Purge all caches. Reload the edit quiz page. Verify that the page loads without errors. The missing question will be shown with a ? icon, and can be removed. (Previously, there was an exception message.)