From the IMAGINiT Technologies Support blog:
You may bump into an error running Revit 2017 with the Autodesk Seek plugin installed.
“Revit cannot run the external application “Autodesk Seek Plugin for Revit”. Contact the provider for assistance. Information they provided to Revit about their identity: Autodesk, www.autodesk.com.
System.NullReferenceException: Object reference not set to an instance of an object.