We are sorry for the inconvenience! Thanks very much for reporting this problem.
It has just been fixed, you can download the upated unitypackages here as usual:
Spine Unity Download
Newly created 2D Hinge Chains will no longer have this issue. In case you have saved your scene in the problematic spine-unity version 2020-03-11 that lost the SkeletonRenderer reference, please manually re-assign the SkeletonRenderer reference at the ActivateBasedOnFlipDirection Component. The Component can be found at the hinge chain root GameObject (named <skeletonname> HingeChain Parent <root bone name>).
The issue has been tracked under this ticket id:
https://github.com/EsotericSoftware/spine-runtimes/issues/1643
Thanks again for reporting!