This seems to be a problem introduced by MDL-75513. To replicate:

1. Create a course but don't enrol any students.
2. Go to Gradebook -> User report
3. Notice the following error on the page:

   Warning: Undefined variable $users in /home/jake/moodledata/s401/moodle/grade/report/user/renderer.php on line 150

   Exception - array_keys(): Argument #1 ($array) must be of type array, null givenMore information about this error

   Debug info:

   Error code: generalexceptionmessage

   Stack trace: line 150 of /grade/report/user/renderer.php: TypeError thrownline 150 of /grade/report/user/renderer.php: call to array_keys()line 171 of /grade/report/user/index.php: call to gradereport_user_renderer->user_navigation()