Description of the Bug:
Pressing the "Average Selected Items" button while having no items selected resets the active search and produces no error message.

Steps to reproduce:
1. Go to the "Average Grades" page.
2. Filter the item list using the search bar.
3. Make sure no "Average" checkboxes are ticked.
4. Press the "Average Selected Items" button. (Both top and bottom work for this.)

What should have happened?
Like with having exactly one item selected, the error message "Please select multiple items to average." should appear.

What did happen?
No error message appears, and the active search gets reset.

My System:
PC, Windows 10 (v1909, build 18363.535)
Firefox 71.0 64bit