Fixed missing Null check causing NullPointerExceptions when an SVG image is placed inside a DataGridCheckBoxColumn

[lukas-kirschner]

Pull Request Template

Description

This fixes a missing Null check when setting the image source

Fixes #94

Type of change

Please delete options that are not relevant.

• 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 not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• .NET Framework, Version 4.0
• .NET Framework, Version 4.5
• .NET Framework, Version 4.6
• .NET Framework, Version 4.7
• .NET Framework, Version 4.8
• .NET Core, Version 3.1
• .NET 6 ~ 8
• All Included Samples

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works