Null exception on showing preview in file question located in dynamic panel

[andrewtelnov]

If there is a file question located in a dynamic panel and file downloading/uploading using survey event then on setting survey into preview mode the exception is raised.
The issue was reported on SurveyJS Support Forum.

[andrewtelnov]

We can get similar exception when we press Preview without getting answer from server and get this answer when the question has not a survey for a moment.