Not selecting a matching answer in the Lesson module causes PHP notice.

If you do not choose an answer, but leave the select box at the option 'Choose...' the following PHP notice displays.

Notice: Undefined index: in /var/www/mstorage/i24/moodle/mod/lesson/pagetypes/matching.php on line 188

Notice: Trying to get property of non-object in /var/www/mstorage/i24/moodle/mod/lesson/pagetypes/matching.php on line 188

Replication steps:

1. Create a lesson activity.
2. Add a Question page of type 'Matching'.
3. Fill in a title, description, correct answer response and wrong answer response.
4. Add to the first 'Answer' field 'Blue is a...'.
5. Add to the first 'Matches with answer' field 'Colour'.
6. Add to the second 'Answer' field 'Cold is a...'
7. Add to the second 'Matches with answer' field 'Temperature'.
8. Save the question.
9. Select the 'Preview' icon.
10. On the next page leave the select boxes at 'Choose...'.
11. Submit

Expected result: you get a message telling you to select an option

Actual result: an error is shown