Report this on the Dynamo Github. Its a bug, only I cant determine exactly what causes it. You’ll need to restart Dynamo to fix.
cc @Michael_Kirschner2 - my suspicion is the Document object. If its set to static readonly and the user opens multiple Revit doc’s and Dynamo, then closes the doc Dynamo is attached to, Dynamo attaches to the next open doc - I believe this makes the ‘managed object’ invalid as the Document object isn’t updated. I’ve not been able to reproduce this error consistently though, so the above is anecdotal.