How to reproduce:
1. Create a new project
2. Open UIElements Debugger window (Window->Analysis->UI Elements Debugger)
3. In UIElements Debugger window select any panel from Select a panel dropdown menu
4. Select any element from the opened panels element list
5. In right section click on Show all button
6. Set texture for backgroundImage to anything except "none"
7. Set texture for backgroundImage to none

Expected result: No errors thrown, the texture is set to none
Actual result: NullReferenceException is thrown, the texture isn't removed

Reproducible with: 2019.3.0b6, 2020.1.0a7
Not reproducible with: 2017.4.32f1 (UI Elements debugger not present), 2018.4.10f1, 2019.2.7f2 (different type texture selection button)

Notes: This behavior also reproducible with unity-font