1. What happened

Editor becomes unresponsive and crashes after setting Weight Slider to "Smooth" mode

2. How we can reproduce it using the example you attached

1. Open Assets > Sprites > Fei_Cel_Group in sprite editor
2. Ensure Skinning Editor mode is on
3. Select Weight Slider and change Mode to "Smooth"
4. Double-click different parts of the character a few times
5. Observe the sprite editor become unresponsive (other parts of the character cannot be selected and buttons on the side bar cannot be selected)

Expected Results: Sprite Editor should not become unresponsive
Actual Results: Sprite Editor should becomes unresponsive

Reproducible with: 2019.3.0b3 (2D Animation 3.0.4), 2020.1.0a4 (2D Animation 3.0.5)

Note: Selecting other sprites ("Square") in Sprites folder or selecting a game object ("Main Camera") in Hierarchy tab resolves unresponsive behavior