Steps to reproduce:
1. Open attached project
2. Press on the particle system
3. In the Inspector double-click "Start Color" to bring up the Gradient editor
4. In the gradient, editor double click bottom right "gradient marker"(While double-clicking, make sure to HOLD the mouse button on the second click)
5. While still holding the mouse button down remove selected "gradient marker" by dragging away from the Gradient
6. Release the mouse button and select any color in the Color Picker

Expected result: Editor doesn't let the user select the color because gradient arrow is removed.
Actual result: NullReferenceException is thrown when trying to select a color.

Reproduced in: 2017.4.20f2, 2018.3.6f1,2019.1.0b3, 2019.2.0a6