An error is displayed when accessing a chat as a guest

As found when running MDLQA-16736, if you log in as a guest and go to a course with guest access and access a chat activity, an error message is displayed:

line 1553 of /lib/questionlib.php: call to moodle_page->__get()
line 4690 of /lib/navigationlib.php: call to question_extend_settings_navigation()
line 4309 of /lib/navigationlib.php: call to settings_navigation->load_course_settings()
line 819 of /lib/pagelib.php: call to settings_navigation->initialise()
line 939 of /lib/pagelib.php: call to moodle_page->magic_get_settingsnav()
line 135 of /blocks/settings/block_settings.php: call to moodle_page->__get()
line 341 of /blocks/moodleblock.class.php: call to block_settings->get_content()
line 235 of /blocks/moodleblock.class.php: call to block_base->formatted_contents()
line 1222 of /lib/blocklib.php: call to block_base->get_content_for_output()
line 1280 of /lib/blocklib.php: call to block_manager->create_block_contents()
line 377 of /lib/blocklib.php: call to block_manager->ensure_content_created()
line 4065 of /lib/outputrenderers.php: call to block_manager->region_has_content()
line 28 of /theme/classic/layout/columns.php: call to core_renderer->blocks()
line 1472 of /lib/outputrenderers.php: call to include()
line 1398 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line ? of unknownfile: call to core_renderer->header()
line 1980 of /lib/setuplib.php: call to call_user_func_array()
line 64 of /mod/chat/view.php: call to bootstrap_renderer->__call()