Hi Paul,
Thank you for contacting Syncfusion support.
We can reproduce the reported exception issue while trying to filter the NumericColumn with special characters in FilterRow and confirmed it as a defect. We have logged a defect report for this and fix for this issue will be included in our 2018 Volume 3 SP1 release which is expected to be rolled out by end of October 2018.
Regards,
Deivaselvan