I'm getting an error when I try to grade an assignment.
If you open an assignment as a teacher, then click on "View all submissions" and then the button "GRADE", a new page opens up with the error saying "undefined".

This is what I get in Apache's error.log:

[Thu Dec 29 11:09:43.518333 2016] [:error] [pid 7171] [client 185.18.60.220:50911] Default exception handler: Coding error detected, it must be fixed by a programmer: $PAGE->context was not set. You may have forgotten to call require_login() or $PAGE->set_context() Debug: \nError code: codingerror\n* line 485 of /lib/pagelib.php: coding_exception thrown\n* line 1572 of /lib/pagelib.php: call to moodle_page->magic_get_context()\n* line 674 of /lib/pagelib.php: call to moodle_page->initialise_theme_and_output()\n* line 817 of /lib/pagelib.php: call to moodle_page->magic_get_theme()\n* line 26 of /theme/uikit/renderers.php: call to moodle_page->_get()\n* line 583 of /lib/outputlib.php: call to include_once()\n* line 496 of /lib/outputlib.php: call to theme_config->_construct()\n* line 7130 of /lib/moodlelib.php: call to theme_config::load()\n* line 417 of /lib/classes/user.php: call to get_list_of_themes()\n* line 578 of /lib/classes/user.php: call to core_user::fill_properties_cache()\n* line 1015 of /user/externallib.php: call to core_user::get_property_type()\n* line 2785 of /mod/assign/externallib.php: call to core_user_external::user_description()\n* line ? of unknownfile: call to mod_assign_external::get_participant_returns()\n* line 124 of /lib/externallib.php: call to call_user_func()\n* line 185 of /lib/externallib.php: call to external_api::external_function_info()\n* line 59 of /lib/ajax/service.php: call to external_api::call_external_function()\n, referer: https://moodledomain/mod/assign/view.php?id=462&rownum=0&action=grader&userid=36