Crash: Loading a new level from the editor "(level) still around trying to load"

I’m getting this now loading one specific umap out of many (others seem to work). This is after moving too 4.9 Preview 3.

I don’t have any blueprint tabs open.

It’s not quite 100% - perhaps 80%.

Recent output:

[2015.08.12-18.42.26:219][219]LogReferenceChain: Generating reference chains …
[2015.08.12-18.42.26:220][219]LogReferenceChain:
[2015.08.12-18.42.26:221][219]LogReferenceChain: External Referencers:
[2015.08.12-18.42.26:222][219]LogReferenceChain:
[2015.08.12-18.42.26:224][219]LogReferenceChain: (standalone) World /Game/Levels/HintScreenScene.HintScreenScene->CurrentLevel
The thread 0x2290 has exited with code 0 (0x0).
[2015.08.12-18.42.53:574][219]LogReferenceChain: Level /Game/Levels/HintScreenScene.HintScreenScene:PersistentLevel->UE4Editor-Engine.dll!ULevel::AddReferencedObjects() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.9\engine\source\runtime\engine\private\level.cpp:255]
[2015.08.12-18.42.53:576][219]LogReferenceChain: AbstractNavData /Game/Levels/HintScreenScene.HintScreenScene:PersistentLevel.AbstractNavData-Default->CachedWorld
[2015.08.12-18.42.53:577][219]LogReferenceChain: (target) World /Game/Levels/DefaultLevel.DefaultLevel
Fatal error: [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.9\Engine\Source\Editor\UnrealEd\Private\EditorServer.cpp] [Line: 1773]
/Game/Levels/DefaultLevel.DefaultLevel still around trying to load …/…/…/…/…/…/Users//Documents/Unreal Projects/MM_04/Content/Levels/HintScreenScene.umap
World /Game/Levels/DefaultLevel.DefaultLevel [target]
AbstractNavData /Game/Levels/HintScreenScene.HintScreenScene:PersistentLevel.AbstractNavData-Default (ObjectProperty /Script/Engine.NavigationData:CachedWorld)
World /Game/Levels/HintScreenScene.HintScreenScene (standalone) (ObjectProperty /Script/Engine.World:PersistentLevel)

UE4Editor.exe has triggered a breakpoint.