Error when attempting to access a glossary as a guest

As found by dag in MDLQA-16250:

Steps to reproduce:

1. Enable the log in as a guest button and guest access in a course.
2. Create a glossary activity in the course.
3. Log out then log in as a guest and go to the glossary.

Expected result: The guest can view the glossary activity.

Actual result: An error message is displayed.

Exception - mod_glossary\output\standard_action_bar::create_add_button(): Return value must be of type stdClass, string returned

More information about this error
Debug info:
Error code: generalexceptionmessage
Stack trace:

line 160 of /mod/glossary/classes/output/standard_action_bar.php: TypeError thrown
line 107 of /mod/glossary/classes/output/standard_action_bar.php: call to mod_glossary\output\standard_action_bar->create_add_button()
line 37 of /mod/glossary/classes/output/renderer.php: call to mod_glossary\output\standard_action_bar->export_for_template()
line 315 of /mod/glossary/view.php: call to mod_glossary\output\renderer->main_action_bar()