I was using the Unity editor and had saved my project and ran (Play) in focused mode with no errors. I went to grab a drink and came back to this error in my Console.
This error is discussed on this Unity Forum Page
The ‘quick fix’ for this is to restart your Unity Editor.
Other more technical fixes were related to the BlendTree Animations:
It happened to me when copying a blend tree from one animation controller to another and after opening/double-clicking the blend tree to make the edits. Found that the blend tree graph in inspector was empty. So toggled the blend type to another and back to what it was and the errors disappear.
Quote Source/Credit: https://forum.unity.com/members/ibx00.1897043/
I hope this helps others that run into this error unexpectedly.
