As an administrator, you are allowed to uninstall the tool_usertours plugin, but if you are using the boost theme, this fails with a database error. This also appears to break Moodle such that all pages subsequently fail with this error. The error message is:

Debug info: Table 'moodle.mdl_tool_usertours_tours' doesn't exist
SELECT t.*
FROM mdl_tool_usertours_tours t
WHERE t.enabled = 1
AND t.id IN (SELECT s.tourid FROM mdl_tool_usertours_steps s GROUP BY s.tourid)
ORDER BY t.sortorder ASC
[array (
)]
Error code: dmlreadexception
Stack trace:

line 486 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 1216 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
line 69 of /admin/tool/usertours/classes/cache.php: call to mysqli_native_moodle_database->get_records_sql()
line 82 of /admin/tool/usertours/classes/cache.php: call to tool_usertours\cache::get_enabled_tourdata()
line 580 of /admin/tool/usertours/classes/manager.php: call to tool_usertours\cache::get_matching_tourdata()
line 565 of /admin/tool/usertours/classes/manager.php: call to tool_usertours\manager::get_matching_tours()
line 505 of /admin/tool/usertours/classes/helper.php: call to tool_usertours\manager::get_current_tour()
line 75 of /admin/tool/usertours/lib.php: call to tool_usertours\helper::bootstrap()
line 1279 of /lib/outputrenderers.php: call to tool_usertours_before_footer()
line 437 of /lib/outputrenderers.php: call to core_renderer->footer()
line 226 of /admin/renderer.php: call to plugin_renderer_base->__call()
line 679 of /admin/index.php: call to core_admin_renderer->upgrade_plugin_check_page()

To reproduce:

1. Install Moodle and leave the boost theme as the default
2. Go into Administration -> Plugins -> Plugins overview
3. Find tool_usertours and uninstall it
4. The error message appears
5. Try to go to another page in the site (e.g. /courses/view.php?id=2)
6. The error message appears and the page is not shown