That is definitely a bug. At first, I could not reproduce it because I was always going through the popup dialog (double-click data grid). If I go through the property grid and use the "..." button, I get the same error as you do. So, there is a workaround and we will fix it ASAP.
Update: found and fixed. The fix will be part of 6.5.0 and in the meantime you can use the workaround. Sorry for the inconvenience :(
Same error here, I submitted a support ticket for this error yesterday. The error is thrown when attempting to edit the XPath constraint on a data grid with 'XPath' as data source, no matter what you put into the constraint (even if it's empty).