'UnityEngine.Rendering.ProbeVolumeLightingTab' is missing the class attribute 'ExtensionOfNativeClass'!

I have installed a fresh version of Unity6 (6000.0.37f1). I imported a small package I had created in Unity 5. Upon running my scene I experienced the above error. (The built scene runs correctly)

I thought it might be down to my imported package, so I created a new empty scene, and ran that - I still get the above error.

Could someone please point me in the right direction to resolve this warning?

Thanks

D.