admin_externalpage is in the incorrect namespace

[sharpchi]

In classes/navigation.php this line:

moodle-block_multiblock/classes/navigation.php

Line 162 in 889782a

} else if ($page instanceof admin_externalpage) {

has the class admin_externalpage which doesn't have the correct namespace. It could either be \admin_externalpage or use use admin_externalpage; at the top.

Found the error running phpstan. I don't know if this is a code path that's ever reached.