This very strange because article's object must have such property. You can remove this notice by replacing in line 108:
if ($article->access <= $user->get('aid', 0)) {
with
if (@$article->access <= $user->get('aid', 0)) {
But this modification will hide this notice but will not fix the problem. Actually I do not know why JComments plugin is called with the incomplete object. Any case this code must not produce a 500 error because this is only notice not fatal error. Try to make change I have suggested and write here about results.