Why Editor throws Null Reference Exception when changing a transforms position while animation clips rec mode?

I'm using unity 2018.4.0, I try to change some transforms position for creating body-parts based animations.

My characters body parts are seperate objects attached with sprite renderer components for example: leg, knee, arm, and parent gameobject with Animator component.

The thing is The Editor Throws NullReferenceException when changing some body parts position during animation clip recording mode, for this reason, objects position always resets at clips starting point, instead of returning to the point where I put in the first place. And Objects positions are reset even when not in rec mode.