Good news, I figured it out: If an object is marked as static it is not allowed to access its vertices. It's that simple. I just turned off static and now it's working just fine. Thanks so much, and great product! 

Just another question. The trees I'm using are from the asset store Realistic Tree Pack 1. This is the full error:

not allowed to access vertices on the mesh 'Combined Mesh (root: scene)(clone)' (isReadable is false; Read/Write must be enabled in import settings)

Is it possible that when importing the asset package that a read write can be enabled somewhere. I know this can be done with imported FBX, but these are tree creator trees and I can't find this option for these trees? Thanks

Wow, thanks for you speedy reply, appreciated. I'll follow your steps and let you know what I find out.