Apologies if this issue has already been reported. I searched and could not find it.

PHP catchable fatal error gets thrown when the following steps are followed:

Create new assignment.
Set 'Require students click submit button' to yes.
Go to assignment, click 'View/grade all submissions'.
In Edit the column for a student who has not yet made a submission, choose the action 'Prevent submission changes'. Causes error.

Should be an alert that there is no submission yet or something a little more helpful to end user.

The error is:
Argument 1 passed to assign_submission_plugin::lock() must be an instance of stdClass, boolean given, called in [dirroot]/mod/assign/locallib.php on line 4464 and defined