If tags are enabled site wide (site administration -> advanced features), but are disabled for courses (site administration -> appearance -> manage tags), and course settings are saved, you get a warning and notice because $data ->tags is undefined. 

Steps to reproduce: 

1. Turn on debugging
2. Go to Site administration -> Advanced features 
3. Make sure tags functionality is enabled
4. Go to Site administration -> Appearance -> Manage tags 
5. In the tags area, find the Courses row and disable it
6. Create a course and go to it
7. Edit setting
8. Save and display

Output: 

Notice: Undefined property: stdClass::$tags in /var/www/html/course/lib.php on line 2570

Warning: array_values() expects parameter 1 to be array, null given in /var/www/html/course/lib.php on line 2570

Notice: Undefined property: stdClass::$tags in /var/www/html/course/lib.php on line 2571

Expected: 

Not see a notice or warning when saving a course. 

Related issue: 

https://tracker.moodle.org/browse/MDL-66446

Looks like this was reported and partially fixed before. That fix added a check to the if statement in the code on line 2568 in course/lib.php for the site wide tags setting, but we are still missing a solution for the other way to disable course tags (setting under site administration -> appearance -> manage tags). 

Potential solution: 

Replace the check for tags being enabled on line 2568 of course/lib.php  

} else if ($field == 'tags' && !empty($CFG->usetags)) {

with

} else if ($field == 'tags' && isset($data->tags)) {

or potentially add this check to the current line if the other part is still needed for something. Or maybe add something else so that it includes a check for the manage tags setting. 

The reasoning behind this suggestion though is that further below in the same function (update_course), the part that actually updates the tags in the course is wrapped in a check for isset($data->tags). So it would make sense that the code checking if it should add tags to $updatedfields would also use the same criteria as the part that actually updates them.