How to reproduce:
1. Initialize Unity Editor with Platform Toolkit for Steam and Unity Services Steam Authentication
2. Initialize Platform Toolkit
3. Add steamworks media to the project to call Steam API functions
4. Observe how the code doesn't cope with callbacks that it didn't initiate

Actual result: Combining both Platform Toolkit for Steam and Unity Services Steam Authentication causes a NullReferenceException
Expected result: Both Platform Toolkit for Steam and Unity Services Steam Authentication should work independently and together

Could not test: Project is not reproducible, was not tested on any versions or platforms. The user has also not attached the version they were using

Note: This issue was not tested by CQA