This specific error, coming from this location (eg, NOT your code, such as UnityEditor.Graphs
or PlasticSCM as in your case above), is often fixed by either Reset Layouts (upper right) or else Right-click Project, Reimport-All.
Extra unwanted packages in new projects (collab, testing, rider and other junk):
Sometimes the package system gets borked from all this unnecessary churn and requires the package cache to be cleared:
Otherwise, if the error is in your code, it’s always the same answer:
The answer is always the same… ALWAYS!
How to fix a NullReferenceException error
https://forum.unity.com/threads/how-to-fix-a-nullreferenceexception-error.1230297/
Three steps to success:
- Identify what is null ← any other action taken before this step is WASTED TIME
- Identify why it is null
- Fix that