Null exception when clicking on Active Tilemap dropdown while using custom brush script

[2D][Tilemap] Null exception when clicking on Active Tilemap dropdown while using custom brush script

Repro steps:

• Select Grid Information Brush in Tile Palette window
• Click on "Create New Tilemap" dropdown in Tile Palette
• Observe dropdown does not open and null ref exception in console

Expected result: Dropdown opens and no errors in console
Actual result: Dropdown does not open and null ref errors in console