How to reproduce:
1. Open the attached 'case_1316957' project
2. Open the 'Test' Scene
3. In the Project window right-click on the StandardTextureBlend shader and select Create > Material
4. Drag the newly created Material onto the Cube
5. In the top bar menu select Tools > Polybrush > Polybrush Window
6. In the Hierarchy select the Cube
7. In the Polybrush Window click on the 'Paint texture on meshes' button
8. Click on the 'Configure' button located next to the Material dropdown
9. Make sure that the Channel of the _MainTex is set to 'UV2'
10. Press the 'Save' button
11. Observe the Console

Expected result: The configurations get saved, no visible errors present
Actual result: Multiple errors appear in the Console

Reproducible with: Polybrush 1.0.2-preview.1, 1.1.0-pre.1 (2018.4.32f1, 2019.4.21f, 2020.2.6f1, 2021.1.0b8, 2021.2.0a6)
Not reproducible with: Polybrush 1.0.1 (2018.4.32f1, 2019.4.21f, 2020.2.6f1, 2021.1.0b8, 2021.2.0a6)

Notes:
- The following errors get thrown in the Console:
KeyNotFoundException: The given key was not present in the dictionary.
NullReferenceException: Object reference not set to an instance of an object.

- Sometimes after pressing the 'Configure' button the Console fills up with NullReferenceException or ArgumentOutOfRangeException exceptions and the _MainTex configuration section is not displayed
- Sometimes the "KeyNotFoundException: The given key was not present in the dictionary." error gets thrown after pressing the 'Configure' button