Blendtree slider handle disappears when the Motion Threshold value is changed by dragging the Threshold parameter in Motion diagram.

Steps to repro:
1. Create a new project > Create a Cube game object.
2. Windows > Animation > Animation > Click on Create button.
3. Project Window > Open Cube Animator > Create Blend Tree.
4. Open the Blend Tree > Right Click > Add > Add Motion & Blend Tree.
5. Drag & change the Threshold value in the motion diagram.
6. Select Parent Blend Tree and move its slider.

Actual Result:
Blendtree slider handle disappears.

Expected Result:
Blendtree slider handle should not disappear.

Reproducible in:
2020.2.0a17, 2020.1.0b15, 2019.4.3f1, 2019.3.15f1, 2019.2.1f1, 2019.1.0f1, 2018.4.23f1, 2019.3.0a1.

Working fine in:
2018.2.0f2.

Environment:
Occurring on Windows 10 & Mac 10.15.