Fix disposing a modal after hiding error

Created by: ernestob

Description

Checking if the focus property has been set before reading it.

Motivation & Context

When you dispose a modal after hiding it, it throws an error.

Type of changes

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

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

Related issues

Fixes #37409 (closed).