[AI] Editor crash when moving BT nodes if the BB has a BlackboardKeyType_Object with a null BaseClass

The editor crashes if a BT is changed while its Blackboard has an Object type key with its BaseClass set to None (null). There is no warning or log alerting the user that setting the BaseClass to None is not supported/advised.

1. Create a new Blackboard called BBTest
2. Create a new BehaviorTree call BTTest
3. In BTTest, set the Blackboard Asset to be BBTest
4. Add a Selector node from the Root node of BTTest
5. Add a Wait task from the Selector
6. Save the BT
7. In BBTest, add a new Object key called NullBaseClass
8. Select NullBaseClass and in the Details panel, expand the KeyType section
9. In BaseClass, select None
10. Save BBTest
11. In BTTest, move the Wait node

Expected Result: The node moves with the cursor as normal.

Actual Result: The editor will crash with a callstack containing UBlackboardKeyType_Object::IsAllowedByFilter