When a teacher launch pdf editor in an assignment with access restriction for students the teacher get an error and can't open pdf editor. 

The trace stack that gives the error is:

Error code: codingerror* line 453 of /lib/pagelib.php: coding_exception thrown* line 773 of /lib/pagelib.php: call to moodle_page->magic_get_context()* line 1437 of /lib/weblib.php: call to moodle_page->_get()* line 1396 of /lib/grade/grade_item.php: call to format_string()* line 192 of /availability/condition/grade/classes/condition.php: call to grade_item->get_name()* line 145 of /availability/condition/grade/classes/condition.php: call to availability_grade\condition::get_cached_grade_name()* line 159 of /availability/classes/condition.php: call to availability_grade\condition->get_description()* line 526 of /availability/classes/tree.php: call to core_availability\condition->get_standalone_description()* line 476 of /availability/classes/tree.php: call to core_availability\tree->get_full_information_recursive()* line 216 of /availability/classes/info.php: call to core_availability\tree->get_result_information()* line 1801 of /lib/modinfolib.php: call to core_availability\info->is_available()* line 1830 of /lib/modinfolib.php: call to cm_info->obtain_dynamic_data()* line 1162 of /lib/modinfolib.php: call to cm_info->get_user_visible()* line 2836 of /lib/moodlelib.php: call to cm_info->_get()* line 481 of /lib/externallib.php: call to require_login()* line 2822 of /mod/assign/externallib.php: call to external_api::validate_context()* line ? of unknownfile: call to mod_assign_external::get_participant()* line 229 of /lib/externallib.php: call to call_user_func_array()* line 64 of /lib/ajax/service.php: call to external_api::call_external_function()