I cam accross a strange error
I am using this piece of xaml
and everything worked fine until I added another radtreeview with the same Hierarchy converter then i got the message mention in the title. I fixed it by registrating the converter twice
and referencing one from one treeview and the other from the other treeview
the converter looks like this
As I have fixed this problem this is not that of a issue for me but I still wanted to let you know about this.