I tried to upgrade from spine-unity-3_7-2019-05-06.unitypackage to spine-unity-3_7-2019-05-28.unitypackage. Whenever I click on a *_SkeletonData.asset file, it just hanging on "Importing small assets" forever. Killed and relaunch the unity editor won't help. Also tried a fresh project checkout and wait the first time reimport-everything, then stuck at the same point.

The log shows the crash on .asset file (since I killed unity).

Hashing assets (6 files)... 0.007 seconds
  file read: 0.001 seconds (0.211 MB)
  wait for write: 0.000 seconds (I/O thread blocked by consumer, aka CPU bound)
  wait for read: 0.006 seconds (CPUT thread waiting for I/O thread, aka disk bound)
  hash: 0.000 seconds
Updating Assets/Sprites/mo01 - GUID: 3ef353e362263ef468c8396690de4653...
 done. [Time: 22.312777 ms]
Updating Assets/Sprites/mo01/mo01.png - GUID: 0c7fd02d4fbd9304a93d5dab9ac50963...
 done. [Time: 34.557669 ms]
Updating Assets/Sprites/mo01/mo01_Material.mat - GUID: ed4edd46fb21a5246a970592559d930e...
 done. [Time: 24.215354 ms]
Updating Assets/Sprites/mo01/mo01_SkeletonData.asset - GUID: 107680d1989cd7341823a46fee1d7e04...
Refresh: detecting if any assets need to be imported or removed ...
A default asset was created for 'Assets/Sprites/mo01/mo01_SkeletonData.asset' because the asset importer crashed on it last time.
You can select the asset and use the 'Assets -> Reimport' menu command to try importing it again, or you can replace the asset and it will auto import again.
UnityEditor.AssetDatabase:Refresh(ImportAssetOptions)
UnityEditor.AssetDatabase:Refresh() (at C:\buildslave\unity\build\artifacts\WinEditor\modules\Editor\AssetDatabaseBindings.gen.cs:306)
Spine.Unity.SkeletonRenderer:FixAllProjectMaterialsStencilCompParameters() (at Assets\Libs\Spine\Runtime\spine-unity\Components\SkeletonRenderer.cs:550)
Spine.Unity.SkeletonRenderer:EditorFixStencilCompParameters() (at Assets\Libs\Spine\Runtime\spine-unity\Components\SkeletonRenderer.cs:536)
Spine.Unity.SkeletonRenderer:AssignSpriteMaskMaterials() (at Assets\Libs\Spine\Runtime\spine-unity\Components\SkeletonRenderer.cs:481)
Spine.Unity.SkeletonRenderer:LateUpdate() (at Assets\Libs\Spine\Runtime\spine-unity\Components\SkeletonRenderer.cs:411)
Spine.Unity.SkeletonRenderer:Initialize(Boolean) (at Assets\Libs\Spine\Runtime\spine-unity\Components\SkeletonRenderer.cs:302)
Spine.Unity.SkeletonAnimation:Initialize(Boolean) (at Assets\Libs\Spine\Runtime\spine-unity\Components\SkeletonAnimation.cs:150)
Spine.Unity.Editor.EditorInstantiation:InstantiateSkeletonAnimation(SkeletonDataAsset, Skin, Boolean) (at Assets\Libs\Spine\Editor\spine-unity\Editor\SpineEditorUtilities.cs:1446)
Spine.Unity.Editor.EditorInstantiation:InstantiateSkeletonAnimation(SkeletonDataAsset, String, Boolean) (at Assets\Libs\Spine\Editor\spine-unity\Editor\SpineEditorUtilities.cs:1419)
Spine.Unity.Editor.SkeletonInspectorPreview:Initialize(Action, SkeletonDataAsset, String) (at Assets\Libs\Spine\Editor\spine-unity\Editor\SkeletonDataAssetInspector.cs:821)
Spine.Unity.Editor.SkeletonDataAssetInspector:InitializeEditor() (at Assets\Libs\Spine\Editor\spine-unity\Editor\SkeletonDataAssetInspector.cs:146)
Spine.Unity.Editor.SkeletonDataAssetInspector:OnEnable() (at Assets\Libs\Spine\Editor\spine-unity\Editor\SkeletonDataAssetInspector.cs:82)
UnityEditor.Editor:CreateEditorWithContextInternal(Object[], Object, Type)
UnityEditor.Editor:CreateEditorWithContext(Object[], Object, Type) (at C:\buildslave\unity\build\Editor\Mono\Inspector\Editor.cs:476)
UnityEditor.Editor:CreateEditor(Object, Type) (at C:\buildslave\unity\build\Editor\Mono\Inspector\Editor.cs:520)
UnityEditor.Editor:CreateEditor(Object) (at C:\buildslave\unity\build\Editor\Mono\Inspector\Editor.cs:515)
UnityEditor.AssetPreviewUpdater:CreatePreviewForAsset(Object, Object[], String) (at C:\buildslave\unity\build\Editor\Mono\AssetPreviewUpdater.cs:28)

[C:\buildslave\unity\build\Modules/AssetDatabase/Editor/V1/CrashedAssetImportsDatabase.cpp line 63]
(Filename: Assets/Libs/Spine/Runtime/spine-unity/Components/SkeletonRenderer.cs Line: 550)

Unity windows version 2019.1.3f1

Rollback to spine-unity-3_7-2019-05-06.unitypackage works for me.