fix(toolbars): Fix error when filtering out duplicate buttons for a button section.

[jbocce]

Context

There was an error introduced in PR #4586 in the ToolbarService.addButtonSection

Changes & Results

Fixed the error.

Testing

Attempt to add buttons to an existing button section. It should work.

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• OS:
• Node version: 20.9.0
• Browser: Chrome 131.0.6778.140

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