Steps to reproduce:
1. Open attached project
2. Select "Run_script_on_this" fbx model in Project view
3. Right click it and select "Setup Animation"
Result: Unity Editor crashes when setting avatar type in OnPreprocessAnimation of an AssetPostprocessor

When debugging an exception is thrown: Exception thrown at 0x00000001416323F4 in Unity.exe: 0xC0000005: Access violation reading location 0x0000000000000000

Reproduced with: 5.5.0b3, 5.5.0p1, 5.6.0b1

Not reproducible with: 5.4.3p4, 5.5.0b2

Regression introduced in 5.5.0b3

Also, user notes that it seems to happen only when changing to the Human rig type.

Callstack:
Unity.exe!mecanim::skeleton::CreateSkeletonPose<struct math::trsX>(struct mecanim::skeleton::Skeleton const ,class mecanim::memory::Allocator &) Unknown
Unity.exe!GenerateMecanimClipsCurves(struct dynamic_array<class AnimationClip ,8>,struct mecanim::animation::AvatarConstant const &,bool,struct HumanDescription const &,class GameObject &,class std::vector<struct AvatarBuilder::NamedTransform,class std::allocator<struct AvatarBuilder::NamedTransform> > const &,class UnityStr const &,bool,class UnityStr &,int) Unknown
Unity.exe!ModelImporter::ImportMuscleClip(class GameObject &,struct mecanim::animation::AvatarConstant const &,class std::vector<struct AvatarBuilder::NamedTransform,class std::allocator<struct AvatarBuilder::NamedTransform> > const &,struct dynamic_array<class AnimationClip ,8>) Unknown
Unity.exe!ModelImporter::GenerateAnimationClips(class GameObject &,class Avatar ,class std::vector<struct AvatarBuilder::NamedTransform,class std::allocator<struct AvatarBuilder::NamedTransform> > const &,class std::basic_string<char,struct std::char_traits<char>,class stl_allocator<char,66,16> > const &,struct ModelImporter::ModelImportData &) Unknown
Unity.exe!ModelImporter::GenerateAll(struct ModelImporter::ModelImportData &) Unknown
Unity.exe!ModelImporter::GenerateAssetData(void) Unknown
Unity.exe!AssetDatabase::ImportAsset(struct UnityGUID,struct Asset &,class std::basic_string<char,struct std::char_traits<char>,class stl_allocator<char,66,16> > const &,class std::basic_string<char,struct std::char_traits<char>,class stl_allocator<char,66,16> > const &,class Unity::Type const ,enum UpdateAssetOptions,class std::basic_string<char,struct std::char_traits<char>,class stl_allocator<char,1,16> > const &,bool,class InstanceIDLocalIdentifierLookup &,struct dynamic_array<class Object ,8> &) Unknown
Unity.exe!AssetDatabase::UpdateAsset(struct UnityGUID const &,struct UnityGUID const &,enum UpdateAssetOptions,struct Hash128,struct AssetImportingTimerStats ) Unknown
Unity.exe!AssetInterface::ProcessAssetsImplementation(double &,enum AssetInterface::CancelBehaviour,enum UpdateAssetOptions) Unknown
Unity.exe!AssetInterface::StopAssetEditing(enum AssetInterface::CancelBehaviour,enum UpdateAssetOptions) Unknown
Unity.exe!AssetInterface::ImportAtPath(class std::basic_string<char,struct std::char_traits<char>,class stl_allocator<char,66,16> > const &,enum UpdateAssetOptions) Unknown
Unity.exe!AssetDatabase_CUSTOM_ImportAsset(struct MonoString ,enum UpdateAssetOptions) Unknown
00000000009b1a1d() Unknown
00000000261681e0() Unknown