Hierarchy v2: Nodes with circular references are unreachable and methods such as GetDepth() will hang

Steps to reproduce:

1. Open attached project and select menu Tools->Hierarchy->Test circular reference
2. Click on option 1 to create normal hierarchy nodes A,B,C and test GetDepth should work fine
3. Now click on attempt to create circular reference by creating A->C->B->A cycle

      4.  Click on  "Call GetDepth() - May Hang"

Actual results: Notice after step 3, Node A,B,C become disconnected from root and are unreachable and click on GetDepth() at this point will cause editor to hang

Reproducible with versions: 6.3  and 4/5/6

Not reproducible with versions: 

Can’t test with versions: Tested on (OS): Mac

Notes:

• This corner case isn't supposed to happen but if a circular reference is ever mistakenly created, issue will still need to be handled.
• This is something we need to handle gracefully if we want to make a robust product.