Applying Undo after using Rectangle Handles in the scene view throws errors in the console

Reproduction steps:
1) Create a New Project or open the project attached below
2) Use Rect Handles (T) on a GameObject in the scene
3) Undo your action (Ctrl + Z or Edit > Undo)
4) Observe the bug in the console

Notes:
- Regression introduced in 5.6.0b1
- Works as expected in 5.6.0a6 (rev. 60b79b74ab2b)
- Issue occurs in 5.6.0b1 (rev. 5229141c7f50)
- Issue occurs in release mode