PHP 7.1 compatibility: using $this in a static method

[danowar2k]

These lines contain a $this reference in a static context:

https://github.com/MaharaProject/moodle-assignsubmission_mahara/blob/87aa4d84d586d97080777bc059e16098f35928ce/classes/observers.php#L121-L135

Found using https://github.com/squizlabs/PHP_CodeSniffer and https://github.com/PHPCompatibility/PHPCompatibility.

Report output:

FILE: ...mod/assign/submission/mahara/classes/observers.php
----------------------------------------------------------------------
FOUND 1 ERROR AFFECTING 1 LINE
----------------------------------------------------------------------
 130 | ERROR | "$this" can no longer be used in a plain function or
     |       | method since PHP 7.1.
----------------------------------------------------------------------

[Richardvi]

I think I'm having the same problem here:

Error code: generalexceptionmessage

• line 130 of /mod/assign/submission/mahara/classes/observers.php: Error thrown
• line 73 of /mod/assign/submission/mahara/classes/observers.php: call to assignsubmission_mahara_observers::release_submitted_view()
• line ? of unknownfile: call to assignsubmission_mahara_observers::submission_graded()
• line 155 of /lib/classes/event/manager.php: call to call_user_func()
• line 75 of /lib/classes/event/manager.php: call to core\event\manager::process_buffers()
• line 834 of /lib/classes/event/base.php: call to core\event\manager::dispatch()
• line 2856 of /mod/assign/locallib.php: call to core\event\base->trigger()
• line 8475 of /mod/assign/locallib.php: call to assign->update_grade()
• line 8621 of /mod/assign/locallib.php: call to assign->apply_grade_to_user()
• line 1589 of /mod/assign/externallib.php: call to assign->save_grade()
• line 261 of /lib/externallib.php: call to mod_assign_external::submit_grading_form()
• line 81 of /lib/ajax/service.php: call to external_api::call_external_function()

When setting the Moodle-Mahara assignment to 'unlock after grading', the error occurs. It will not unlock.