by the PivotGrid. This is caused by line 620 in kendo.pivotgrid.js (Version 2015.1). On this line there is a check for undefined which can be extended to
check for null as well:
value = value !== undefined && value !== null
? value.toString() : value;
I was able to reproduce the error with the sample http://demos.telerik.com/kendo-ui/pivotgrid/local-flat-data-binding
when setting some Fields to null.
It is this behavior intended? For me, it seems like a bug that should be fixed