If the user changes her answer after pressing check, the captivity can end up in a "locked" blocking state, where the user cannot continue.
attached. Do the following steps to recreate:
Get to the second True/False with the text "Is it true?"
Press "Continue", and quickly change the answer to "True".
A empty dialog is shown, and the user is blocked from continuing.
It appears to be a timing problem, where the user has time to change the answer, before getting a "real" retry.
Fix blocking state in IV Adaptivity