NullReferenceException counter keeps incrementing without user input after importing "Unity Anima2D" package

1. Use 2017.1.0a1
2. Download and import "Unity Anima2D" package from the asset store

The counter for following error will keep incrementing even without user input.

NullReferenceException: Object reference not set to an instance of an object
Anima2D.AnimationWindowExtra..cctor () (at Assets/Anima2D/Scripts/Editor/AnimationWindowExtra/AnimationWindowExtra.cs:55)
Rethrow as TypeInitializationException: An exception was thrown by the type initializer for Anima2D.AnimationWindowExtra
System.Runtime.CompilerServices.RuntimeHelpers.RunClassConstructor (RuntimeTypeHandle type) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Runtime.CompilerServices/RuntimeHelpers.cs:101)
UnityEditor.EditorAssemblies.ProcessEditorInitializeOnLoad (System.Type type) (at C:/buildslave/unity/build/Editor/Mono/EditorAssemblies.cs:136)

Suspected cause: Script does not account for the change in Unity versioning from the 5.x series to the new 201x.x.x series.

Occurring on: 2017.1.0a1
Not occurring: 5.6.0b9, 5.5.1f1