Fixes U4-5578 - MNTP with root node, recursive output

Currently when a custom start node is used for the MNTP it renders the child nodes in the dialog.  When expanding one of those nodes, the fact there is a custom start node is again detected, and it's used again, leading to the same nodes being displayed.

This amend checks that the currently requested node isn't already under the start node - if it is, then the switch to use the custom start node isn't made.