Steps to reproduce:

1. Create a course (Don't enrol any user)
2. Click Grades and then select "User report"
3. Set "Select all or one user" to "All users(0)"
4. You will observe fatal error.

Problem is reports object is not created if no user is present and you are viewing all grades.

Also, while on this part of code you will observe that following code is only placed in last so it will not trigger report->viewed event for each user, while viewing all the users.

// Trigger report viewed event.

$report->viewed();