Editor crash when modifying Skeletal Animation Section of an unsaved additive animation

Unsaved animations can cause the editor to crash if they are in a sequence, if the sequence evaluates them after an edit. 

1. Create a Level Sequence
2. Add a Mannequin to the sequence
3. Track an animation on the mannequin using one of the existing animations (e.g. ThirdPersonWalk)
4. Open the animation, and edit an existing curve. Note the message that says the animation is being edited, but do not click apply
5. Go back to the sequence and move the animation section
6. Go back to the animation and in the Additive Settings set the Additive Anim Type to local and save
7. Edit the curve again, and do not apply or save
8. Go back to the sequence and move the animation section

RESULT

The editor will crash. With the following assertion:

Assertion failed: InCurveToInitFrom.UIDToArrayIndexLUT != nullptr [Link Removed] [Line: 324]

Assertion failed: InCurveToInitFrom.UIDToArrayIndexLUT != nullptr [Link Removed] [Line: 324]

0x00007ffb9cfad759 KERNELBASE.dll!UnknownFunction []
0x00007ffb330899e6 UE4Editor-Core.dll!ReportAssert() [D:\_Release-4.26\Engine\Source\Runtime\Core\Private\Windows\WindowsPlatformCrashContext.cpp:1627]
0x00007ffb3308d1c8 UE4Editor-Core.dll!FWindowsErrorOutputDevice::Serialize() [D:\_Release-4.26\Engine\Source\Runtime\Core\Private\Windows\WindowsErrorOutputDevice.cpp:78]
0x00007ffb32d9e40d UE4Editor-Core.dll!FOutputDevice::LogfImpl() [D:\_Release-4.26\Engine\Source\Runtime\Core\Private\Misc\OutputDevice.cpp:61]
0x00007ffb32d34ba5 UE4Editor-Core.dll!AssertFailedImplV() [D:\_Release-4.26\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:104]
0x00007ffb32d36c60 UE4Editor-Core.dll!FDebug::CheckVerifyFailedImpl() [D:\_Release-4.26\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:461]
0x00007ffb2fbab9cc UE4Editor-Engine.dll!FBaseBlendedCurve<TMemStackAllocator<0> >::InitFrom() [D:\_Release-4.26\Engine\Source\Runtime\Engine\Public\Animation\AnimCurveTypes.h:324]
0x00007ffb2fdba455 UE4Editor-Engine.dll!UAnimSequence::GetBonePose_Additive() [D:\_Release-4.26\Engine\Source\Runtime\Engine\Private\Animation\AnimSequence.cpp:1766]
0x00007ffb2fdb7bdf UE4Editor-Engine.dll!UAnimSequence::GetAnimationPose() [D:\_Release-4.26\Engine\Source\Runtime\Engine\Private\Animation\AnimSequence.cpp:1505]
0x00007ffb27a8e36e UE4Editor-MovieSceneTracks.dll!UMovieSceneSkeletalAnimationSection::GetRootMotionTransform() [D:\_Release-4.26\Engine\Source\Runtime\MovieSceneTracks\Private\Sections\MovieSceneSkeletalAnimationSection.cpp:493]
0x00007ffb27add4e7 UE4Editor-MovieSceneTracks.dll!UMovieSceneSkeletalAnimationTrack::SetUpRootMotions() [D:\_Release-4.26\Engine\Source\Runtime\MovieSceneTracks\Private\Tracks\MovieSceneSkeletalAnimationTrack.cpp:538]
0x00007ffb24695711 UE4Editor-Sequencer.dll!FMoveKeysAndSections::OnDrag() [D:\_Release-4.26\Engine\Source\Editor\Sequencer\Private\Tools\EditToolDragOperations.cpp:841]
0x00007ffb2469a31c UE4Editor-Sequencer.dll!FSequencerEditTool_Movement::OnMouseMove() [D:\_Release-4.26\Engine\Source\Editor\Sequencer\Private\Tools\SequencerEditTool_Movement.cpp:93]
0x00007ffb24654f3f UE4Editor-Sequencer.dll!FSequencerInputHandlerStack::ProcessEvent() [D:\_Release-4.26\Engine\Source\Editor\Sequencer\Private\SequencerInputHandlerStack.h:86]
0x00007ffb2463bd2c UE4Editor-Sequencer.dll!SSequencerTrackArea::OnMouseMove() [D:\_Release-4.26\Engine\Source\Editor\Sequencer\Private\SSequencerTrackArea.cpp:345]
0x00007ffb2eb72e1a UE4Editor-Slate.dll!<lambda_4d385d0b298919766951e0d5d3094014>::operator()() [D:\_Release-4.26\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:5096]
0x00007ffb2eb54035 UE4Editor-Slate.dll!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_4d385d0b298919766951e0d5d3094014> >() [D:\_Release-4.26\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:378]
0x00007ffb2ebd80d3 UE4Editor-Slate.dll!FSlateApplication::RoutePointerMoveEvent() [D:\_Release-4.26\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:5103]
0x00007ffb2ebc153c UE4Editor-Slate.dll!FSlateApplication::ProcessMouseMoveEvent() [D:\_Release-4.26\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:5594]
0x00007ffb2ebb3a8b UE4Editor-Slate.dll!FSlateApplication::OnMouseMove() [D:\_Release-4.26\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp:5518]
0x00007ffb7a6bbc16 UE4Editor-ApplicationCore.dll!FWindowsApplication::ProcessDeferredMessage() [D:\_Release-4.26\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:2212]
0x00007ffb7a6a9227 UE4Editor-ApplicationCore.dll!FWindowsApplication::DeferMessage() [D:\_Release-4.26\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:2638]
0x00007ffb7a6be3ee UE4Editor-ApplicationCore.dll!FWindowsApplication::ProcessMessage() [D:\_Release-4.26\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:1042]
0x00007ffb7a6a43d0 UE4Editor-ApplicationCore.dll!FWindowsApplication::AppWndProc() [D:\_Release-4.26\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsApplication.cpp:874]
0x00007ffb9f52e858 USER32.dll!UnknownFunction []
0x00007ffb9f52e299 USER32.dll!UnknownFunction []
0x00007ffb7a6bf9f4 UE4Editor-ApplicationCore.dll!FWindowsPlatformApplicationMisc::PumpMessages() [D:\_Release-4.26\Engine\Source\Runtime\ApplicationCore\Private\Windows\WindowsPlatformApplicationMisc.cpp:130]
0x00007ff696088857 UE4Editor-Win64-DebugGame.exe!FEngineLoop::Tick() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:4748]
0x00007ff6960a11dc UE4Editor-Win64-DebugGame.exe!GuardedMain() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\Launch.cpp:169]
0x00007ff6960a12ba UE4Editor-Win64-DebugGame.exe!GuardedMainWrapper() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:137]
0x00007ff6960b529d UE4Editor-Win64-DebugGame.exe!WinMain() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:268]
0x00007ff6960b7fca UE4Editor-Win64-DebugGame.exe!__scrt_common_main_seh() [D:\agent\_work\9\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
0x00007ffb9deb7034 KERNEL32.DLL!UnknownFunction []
0x00007ffb9f75d0d1 ntdll.dll!UnknownFunction []

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