Steps to reproduce :

- import the project attached
- open the scene "animCurves"
- open the animation tab
- select "MrWolf_001" in the Hierarchy
- change to curves in the Animation tab and expand Export_arm_png/Rotation
- selec the Rotation.z channel and notice the curve and the error in the console

NullReferenceException: Object reference not set to an instance of an object
UnityEditorInternal.AnimationWindowState.OnHierarchySelectionChanged (System.Int32[] selectedInstanceIDs)

Explanation :
In unity all rotation curve are stored as quaternion, a quaternion cannot represent rotation greater than 180 degree so if you do a bigger than 180 rotation between 2 keyframe the interpolation will be bad. This is a math limitation of the quaternion.

Editing a quaternion is not trivial so the Animation Windows display euler curve which are converted from the quaternion curves but still the interpolation is done with quaternion.