line 26486 
has this:
data = extend({}, { field: group.field, value: group.value }, group.aggregates[group.field]),
should be this:
data = extend({}, { field: group.field, value: (typeof(group.value) === 'undefined' ? null : group.value) }, group.aggregates[group.field]),
Or something like that.  In any case, without that change, the following will blow up when cell value is undefined.
groupHeaderTemplate: "#=value#"