Reimporting SkeletalMesh with LODs generated via reduction tool crashes when reduction plugin is disabled

When reimporting a Skeletal Mesh that has LODs generated via the reduction tool, the editor will crash if the Skeletal Mesh Simplifier plugin is disabled.

This issue was originally reported on the following UDN: https://udn.unrealengine.com/s/question/0D52L00005PYQupSAH/crash-when-no-skeletal-reduction-plugin-available

1. Open a new project
2. Import a FBX to generate a new Skeletal Mesh
3. Open the mesh in the mesh editor and generate a LOD
4. Save the assets
5. Disable the Skeletal Mesh Simplifier plugin
6. Restart the editor
7. Reimport the Skeletal Mesh
   1. The editor will crash during the import

UE4Editor-SkeletalMeshUtilitiesCommon.dll!FLODUtilities::RegenerateDependentLODs(USkeletalMesh * SkeletalMesh, int LODIndex) Line 2304
at D:\Build++UE4\Sync\Engine\Source\Developer\SkeletalMeshUtilitiesCommon\Private\LODUtilities.cpp(2304)
UE4Editor-MeshBuilder.dll!FSkeletalMeshBuilder::Build(USkeletalMesh * SkeletalMesh, const int LODIndex, const bool bRegenDepLODs) Line 213
at D:\Build++UE4\Sync\Engine\Source\Developer\MeshBuilder\Private\SkeletalMeshBuilder.cpp(213)
UE4Editor-MeshBuilder.dll!FMeshBuilderModule::BuildSkeletalMesh(USkeletalMesh * SkeletalMesh, const int LODIndex, const bool bRegenDepLODs) Line 53
at D:\Build++UE4\Sync\Engine\Source\Developer\MeshBuilder\Private\MeshBuilderModule.cpp(53)
UE4Editor-Engine.dll!FSkeletalMeshRenderData::Cache(const ITargetPlatform * TargetPlatform, USkeletalMesh * Owner) Line 266
at D:\Build++UE4\Sync\Engine\Source\Runtime\Engine\Private\SkeletalMeshRenderData.cpp(266)
UE4Editor-Engine.dll!USkeletalMesh::CacheDerivedData() Line 3563
at D:\Build++UE4\Sync\Engine\Source\Runtime\Engine\Private\SkeletalMesh.cpp(3563)
UE4Editor-Engine.dll!USkeletalMesh::Build() Line 1760
at D:\Build++UE4\Sync\Engine\Source\Runtime\Engine\Private\SkeletalMesh.cpp(1760)
UE4Editor-Engine.dll!USkeletalMesh::PostEditChangeProperty(FPropertyChangedEvent & PropertyChangedEvent) Line 1233
at D:\Build++UE4\Sync\Engine\Source\Runtime\Engine\Private\SkeletalMesh.cpp(1233)
UE4Editor-CoreUObject.dll!UObject::PostEditChange() Line 342
at D:\Build++UE4\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\Obj.cpp(342)
UE4Editor-Engine.dll!FScopedSkeletalMeshPostEditChange::~FScopedSkeletalMeshPostEditChange() Line 361
at D:\Build++UE4\Sync\Engine\Source\Runtime\Engine\Private\SkeletalMesh.cpp(361)
UE4Editor-UnrealEd.dll!UReimportFbxSkeletalMeshFactory::Reimport(UObject * Obj, int SourceFileIndex) Line 6690
at D:\Build++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\Factories\EditorFactories.cpp(6690)
UE4Editor-UnrealEd.dll!FReimportManager::Reimport(UObject * Obj, bool bAskForNewFileIfMissing, bool bShowNotification, FString PreferredReimportFile, FReimportHandler * SpecifiedReimportHandler, int SourceFileIndex, bool bForceNewFile, bool bAutomated) Line 374
at D:\Build++UE4\Sync\Engine\Source\Editor\UnrealEd\Private\Editor.cpp(374)