InstancedStaticMesh component element mismatch between in-level and BP editor can cause crash when modifying transform in BP editor.
This can happen when a user modified transform in-level actor's InstancedStaticMesh's element then added more element in BP editor and modify the transform of the new element.

Steps to Reproduce

1. Open the attached project
2. (in level)Add a new InstancedStaticMesh instance to the NewBlueprint in the level
3. Open NewBlueprint in Blueprint Editor
4. (in Blueprint Editor)Add a new instance in Blueprint Editor, then transform the newly added instance in Blueprint Editor viewport. This will update the instance in the level, too.
5.(in level) Now edit the transform of the instance in details panel
6. (in Blueprint Editor)Add another instance - this does not add an instance to in level actor
7. (in Blueprint Editor)From Blueprint Editor viewport, move the newly added instance

(steps could be optimized, the important thing is, have a in level actor that has transform modification, then add more elements and edit in BP editor)

UE4Editor_Kismet!FInstancedStaticMeshSCSEditorCustomization::HandleViewportDrag() [instancedstaticmeshscseditorcustomization.cpp:121]
UE4Editor_Kismet!FSCSEditorViewportClient::InputWidgetDelta() [scseditorviewportclient.cpp:462]
UE4Editor_UnrealEd!FEditorViewportClient::UpdateMouseDelta() [editorviewportclient.cpp:2091]
UE4Editor_UnrealEd!FEditorViewportClient::Tick() [editorviewportclient.cpp:1136]
UE4Editor_Kismet!FSCSEditorViewportClient::Tick() [scseditorviewportclient.cpp:168]
UE4Editor_UnrealEd!UEditorEngine::Tick() [editorengine.cpp:1734]
UE4Editor_UnrealEd!UUnrealEdEngine::Tick() [unrealedengine.cpp:396]
UE4Editor!FEngineLoop::Tick() [launchengineloop.cpp:3296]
UE4Editor!GuardedMain() [launch.cpp:166]
UE4Editor!GuardedMainWrapper() [launchwindows.cpp:134]
UE4Editor!WinMain() [launchwindows.cpp:210]
UE4Editor!__scrt_common_main_seh() [exe_common.inl:253]

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Fix Commit3746038
Main Commit3807911
Release Commit3813083
CreatedOct 20, 2017
ResolvedNov 9, 2017
UpdatedApr 27, 2018