Preview of deleted texture is still visible in Sprite Editor Window in Secondary Textures list

Steps to reproduce:

1. Open attached Project
2. Select Project > Assets > Graffiti01 texture > Inspector > Sprite Editor
3. Select Sprite Editor > Secondary Texture
4. Select Secondary Textures > "" > Name DropDown > _MaskTex, Texture > Graffiti02
5. Select Secondary Textures > "" > Name DropDown > _NormalMap, Texture > Graffiti03
6. Select _MaskTex and click on "-"

Expected Result:
Selected texture is deleted and preview is updated with _NormalMap texture

Actual Result:
Deleted texture still remains

Reproducible on: 2020.1.0a19, 2020.1.0a15

Not reproducible on: 2020.1.0a14, 2019.3.0f6, 2019.2.12f1

Note: Secondary Texture option is not available before 2019.2

Environment: Windows 10 and Mac 10.14