All Tile Palette Brush Pick Overlay's columns can be disabled that leads to NullReferenceException errors being spammed

Steps to reproduce:

1. Open the attached project
2. Open the Tile Palette window
3. Click on the button to open the Saved brushes overlay
4. If there are no saved brushes, save one
5. In the overlay window, right-click on the Column names
6. Disable both Columns
7. Observe the Console window

Actual results: A safeguard for disabling all columns is missing, which leads to NullReferenceException errors being spammed in the Console window

Expected results: A safeguard that prevents the disablement of all columns is present

Reproducible with versions: 6000.0.74f1, 6000.3.14f1, 6000.4.5f1, 6000.5.0b6, 6000.6.0a4

Tested on (OS): Windows 11, macOS 26.4.1

Notes:

• Once the bug has been reproduced, it can lead to the Scene view becoming non-responsive (Switch to Game view and then back to Scene view when both windows are docked in the same tab)