[minor] fix in reportview (#3627)

frappe/public/js/frappe/views/reports/reportview.js

@@ -264,7 +264,7 @@ frappe.views.ReportView = frappe.ui.BaseList.extend({
 
 		return {
 			doctype: this.doctype,
-			fields: $.map(this.columns, function(v) { return me.get_full_column_name(v) }),
+			fields: $.map(this.columns || [], function(v) { return me.get_full_column_name(v); }),
 			order_by: this.get_order_by(),
 			add_total_row: this.add_total_row,
 			filters: filters,