At points in our level streaming the AI may be in a level that becomes not shown any more, but it appears the behavior tree may continue to operate. It looks like the navmesh will become invalid at that point and so calls into the navmesh must check for invalid states. Most calls FPImplRecastNavMesh are protected against crashing, but at least one is not. You can see FPImplRecastNavMesh::TestPath does not do a test to see if DetourNavMesh and NavMeshOwner are NULL. Other similar calls are protected, such as the FPImplRecastNavMesh::FindPath. I believe this is a bug and FPImplRecastNavMesh::TestPath should also have this protection.
Hey CombatJack-
If this is affecting your project the best solution would be to enter a pull request on www.github.com to have the change you suggest included in the engine. Doing so will allow you to stay informed of the status of the issue and know if/when the change is accepted into the engine.
Cheers