This may be the exact same issue as http://www.telerik.com/community/forums/silverlight/treelist/error-when-editing-cell-child.aspx
I don't know what the solution was in that post, and I'm doing something a bit weird with an IValueConverter, so I figured it was worth starting a new post. If the code in the above post will solve my problem, please send it my way..
I am using a IValueConverter to push multiple types of objects into my TreeListView. Some properties of the objects are common and some are not. When I try to edit the parent node, it works fine. When I edit the child node, I get a System.Reflection.TargetException.
I have attached a screenshot of the error and a very small/simple project that demonstrates the error. Just expand the parent node and try to click on the child node properties for editing.
Looks like I can't attach the project directly, so here's a link: http://dl.dropbox.com/u/28253/TreeListViewIssue.zip