The Exception is thrown as if the RenderMode is not set to Flat. If the GroupStyleSelector is defined before the RenderMode, the issue is not reproduced.
If an EditorAttribute's EditorStyle is set to None and a custom usercontrol is used as an editor, the DataContext of the usercontrol is not respected. Available in LIB version 2017.1.417 , it will be also available in the R2 2017 Release.
Fixed with LIB version 2015.2.817.
The fix is available with lib version 2015.2.907.
The problem should be resolved with R2 2016.
BindingMode is OneWay when there is editor template and PropertySetMode is None Resolved with LIB version 2014.1.407.