Poll: single/multiple switcher lost the value in case post with errors

How to repeat:

• create new topic: enter some data in the Message, Poll, Options list fields, select "Multiple answers" and leave "topic"field blank;
• click New topic

As a result page with error is opened; all text data are saved, but "single answers" is selected instead of "multiple answers"
It is expected that "Multiple answers" is still selected.