Fixing bug in python-viewer with draw event callable argument

[JuanTheEngineer]

Motivation and Context

Quick fix to Python-Viewer which is currently not running because of an arguments error.

How Has This Been Tested

Types of changes

• Docs change / refactoring / dependency upgrade
• 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 code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[Skylion007]

@JuanTheEngineer This isn't the best way to check for None types. It should have been if ___ is not None relying on the implicit boolean conversion of None is not recommended.

[Skylion007]

As an aside, mypy should have really caught this. Wonder why it did not.