Hello berni94,
I'm glad you like the plugin. Thanks for the report, I am looking into this now. I have not been able to reproduce it for now, but it seems to happen during (de)serialization, i.e. when the plugin is loaded or unloaded. You might be able to get rid of the error if you disable and re-enable the plugin. If that doesn't fix it, restarting the editor might also fix it.
Unfortunately, it is quite difficult to prevent this type of error during serialization, so I would need some more info to find a potential fix for this. When, and how often does this error occur in your project? E.g. does it happen after you rebuild your C# solution, or right after enabling the plugin? Also, how severe is this issue for your workflow?
I'd appreciate if you could update me on these details.