Environment:

Moodle 3.5.2+ (Build: 20181031)
mod_assign 2018051400
mariadb 5.5.56 2
php 7.1.2
CentOS Linux release 7.5.1804
Apache / 2.4.6

Reproduction of the error:
1. the teacher creates an assignment via mod_assign
2. The student submits an assignment and validates it for evaluation
3. The teacher authorizes a new attempt for the student WITHOUT having noted it on his first attempt
4. the student tries to go back to the assignement, the error appears

Fault:
Exception: Argument 1 passed to core_renderer :: user_picture () must be an instance of stdClass, string given, called in [dirroot] /mod/assign/renderer.php on line 1106.

Code causing the error:

In [dirroot] /mod/assign/renderer.php on line +/- 1106:

$ cell2 = new html_table_cell ($ this-> output-> user_picture ($ grade-> grader).

$ this-> output-> spacer (array ('width' => 30)). fullname ($ grade-> grader));

Assumption:
$ grade-> grader is null, empty or different from what is expected because the previous assignment was not rated then there is no grader

Patch:
Removing the call to user_picture makes the problem no longer appears. It's a dirty patch because I'm not able to measure the other implications of the problem:

In [dirroot] /mod/assign/renderer.php on line +/- 1106:

// Graded by.

$ cell1 = new html_table_cell ($ gradedbystr);

// $ cell2 = new html_table_cell ($ this-> output-> user_picture ($ grade-> grader).

// $ this-> output-> spacer (array ('width' => 30)). fullname ($ grade-> grader));

// Nicolas 12/11/18: patch to work around a programming error

$ cell2 = new html_table_cell ($ this-> output-> spacer (array ('width' => 30)). fullname ($ grade-> grader));