Allow nested groups to be removed by nulling the nestedGroups content. (#2852)

Don't update the showNested property if nothing has been set in the data object, unless it is undefined.
Verify group object exists during `onGroupClick`

lib/timeline/component/Group.js

@@ -133,21 +133,34 @@ Group.prototype.setData = function(data) {
   }
 
   if (data && data.nestedGroups) {
-     if (data.showNested == false) {
-      this.showNested = false;
-    } else {
-      this.showNested = true;
+    if (!this.nestedGroups || this.nestedGroups != data.nestedGroups) {
+      this.nestedGroups = data.nestedGroups;
+    }
+    
+    if (data.showNested !== undefined || this.showNested === undefined) {
+      if (data.showNested == false) {
+        this.showNested = false;
+      } else {
+        this.showNested = true;
+      }
     }
 
     util.addClassName(this.dom.label, 'vis-nesting-group');
+    var collapsedDirClassName = this.itemSet.options.rtl ? 'collapsed-rtl' : 'collapsed'
     if (this.showNested) {
-      util.removeClassName(this.dom.label, 'collapsed');
+      util.removeClassName(this.dom.label, collapsedDirClassName);
       util.addClassName(this.dom.label, 'expanded');
     } else {
       util.removeClassName(this.dom.label, 'expanded');
-      var collapsedDirClassName = this.itemSet.options.rtl ? 'collapsed-rtl' : 'collapsed'
       util.addClassName(this.dom.label, collapsedDirClassName);
     }
+  } else if (this.nestedGroups) {
+    this.nestedGroups = null;
+    
+    var collapsedDirClassName = this.itemSet.options.rtl ? 'collapsed-rtl' : 'collapsed'
+    util.removeClassName(this.dom.label, collapsedDirClassName);
+    util.removeClassName(this.dom.label, 'expanded');
+    util.removeClassName(this.dom.label, 'vis-nesting-group');
   }
 
   if (data && data.nestedInGroup) {

lib/timeline/component/ItemSet.js

@@ -1644,7 +1644,7 @@ ItemSet.prototype._onDragEnd = function (event) {
 ItemSet.prototype._onGroupClick = function (event) {
   var group = this.groupFromTarget(event);
 
-  if (!group.nestedGroups) return;
+  if (!group || !group.nestedGroups) return;
 
   var groupsData = this.groupsData;
   if (this.groupsData instanceof DataView) {