Steps to reproduce:

1. Create new project
2. Import 2D package
3. In project tab, go to "Standard Assets/2D/Sprites" folder
4. Select any sprite and in Inspector click "Sprite Editor" button
5. Unity crashes

Alternatively, if sprite editor is open before selecting sprite - Unity would crash as soon as sprite is selected

Reproduced with: 5.6.0b5
Not reproducible: 5.6.0b4

#0 0x007fff91f9db57 in objc_msgSend
#1 0x00000102906cb0 in -[GlobalProgressbarController cleanup]
#2 0x00000120f7fa0a in (wrapper managed-to-native) UnityEditor.EditorUtility:ClearProgressBar () + 0x8a (0x120f7f980 0x120f7fa39) [0x10e77a660 - Unity Child Domain]
#3 0x00000120f7d59f in UnityEditor.SpriteEditorWindow:RefreshRects () + 0xddf (0x120f7c7c0 0x120f7d68e) [0x10e77a660 - Unity Child Domain]
#4 0x00000120f28d99 in UnityEditor.SpriteEditorWindow:OnEnable () + 0x5b9 (0x120f287e0 0x120f28e44) [0x10e77a660 - Unity Child Domain]
#5 0x000001256df39a in (wrapper runtime-invoke) object:runtime_invoke_void__this__ (object,intptr,intptr,intptr) + 0xfa (0x1256df2a0 0x1256df4cd) [0x10e77a660 - Unity Child Domain]