Fixed broken barCharts with handleOverlap=='stack'

lib/timeline/component/LineGraph.js

@@ -820,7 +820,8 @@ LineGraph.prototype._updateYAxis = function (groupIds, groupRanges) {
   if (groupIds.length > 0) {
     // this is here to make sure that if there are no items in the axis but there are groups, that there is no infinite draw/redraw loop.
     for (var i = 0; i < groupIds.length; i++) {
-      if (this.groups[groupIds[i]].options.yAxisOrientation == 'left') {
+      var group = this.groups[groupIds[i]];
+      if (group && group.options.yAxisOrientation == 'left') {
         yAxisLeftUsed = true;
         minLeft = 0;
         maxLeft = 0;