fix: guard against accidental undefined access in Brush

[ckifer]

Description

Fix brush bug experienced in current release, guard against it in 3.x too

Related Issue

This is interesting... not sure why people use Brush like this? Asked for a reason in the comment
#2093 (comment)

Motivation and Context

bug prevention

How Has This Been Tested?

it hasn't really, but it doesn't hurt anything

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have added a storybook story or extended an existing story to show my changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (3.x@8b67483). Click here to learn what that means.

Additional details and impacted files

@@          Coverage Diff           @@
##             3.x    #4504   +/-   ##
======================================
  Coverage       ?   95.42%           
======================================
  Files          ?      111           
  Lines          ?    21559           
  Branches       ?     2985           
======================================
  Hits           ?    20572           
  Misses         ?      981           
  Partials       ?        6           

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[ckifer]

nothing really to review here, gonna ship it