Removed node scoping

Removed scopes from NestedSet calculation

v5.0.1: Modified assertNodeExists to perform null check

I've suddenly started running into issues where the root folder (which now has an _lft value of 0, and an _rgt value of 0) forces the assertNodeExists check to fail.

To better perform this test, we should check if the value returned from the models attribute is null as this is the value returned if the node doesn't exist.