This problem arises from #37807 by @chmst

Steps to reproduce the issue

select a user debug permissions
in the filter options select start level 5 and send level 6

Expected result

no idea really as I have no idea what level 4,5,6 are supposed to do but at a minimum there should be no errors

Actual result

Warning
: Undefined array key 0 in
administrator\components\com_users\tmpl\debuguser\default.php
on line
40


Warning
: Attempt to read property "checks" on null in
administrator\components\com_users\tmpl\debuguser\default.php
on line
40


Warning
: Trying to access array offset on value of type null in
administrator\components\com_users\tmpl\debuguser\default.php
on line
40

The error can be solved by changing line 40 to
$check = isset($this->items[0]->checks[$name]);

But I suspect it is more than that else this code will never be reached with that change