Discovered by meowmiomew and myself while testing MDLQA-19249.

There are a couple of issues:

1) On step 5 I'm not able to filter by a tag, even though it's showing in the related question:

And inspecting the browser console I can see a JS error:

Note: This issue has been already reported in https://tracker.moodle.org/browse/MDL-82477.

2) An error occurs when trying to create a random question in a quiz from a 'top' category that includes subcategories with tagged questions.

Steps to reproduce the problem:

1. Create a new course and go to the question bank.
2. Select "Categories" in the menu and add a category (cat1, cat2) in the "Top for course" category:
3. On each category, create two true/false questions (cat1-q2, cat1-q2, cat2-q1, cat2-q2)
4. For each question, click on "Edit > Manage tags" and add t1 tag to cat1-q1 and cat2-q1 and t2 tag to cat1-q2 and cat2-q2
5. Back to the course, create a Quiz and click "Add question"
6. Click on "Add > a random question"
7. Select "Top for course" from the category filter and click "Apply filters".
8. Click "Add condition" to filter by tag.
9. Select t1 and click "Apply filters".
10. Click "Add random question".
11. Result: The following error is displayed: