Editor freezes on
-Adding branch group in the Tree root node.
-Double-clicking on Tree Root Node.

Steps to repro:
1. Create a new Project.
2. GameObject > 3D > Tree.
3. In Inspector > Select Tree Root Node > Add Branch Group.

Actual Result:
Editor freezes on adding branch group in Tree.

Expected Result:
New branch group should be added.

Reproducible in:
2020.1.0a17, 2020.1.0a15.

Working Fine in:
2020.1.0a14, 2019.3.0f5 (9134dab13572)

Environment:
Occurs on Windows and Mac.