Fixed a group "null" being displayed in IE when not using groups (see also #238)

HISTORY.md

@@ -18,6 +18,7 @@ http://visjs.org
 ### Timeline
 
 - Fixed items in groups sometimes being displayed but not positioned correctly.
+- Fixed a group "null" being displayed in IE when not using groups.
 
 ### Network
 

lib/timeline/component/Group.js

@@ -78,11 +78,11 @@ Group.prototype.setData = function(data) {
   if (content instanceof Element) {
     this.dom.inner.appendChild(content);
   }
-  else if (content != undefined) {
+  else if (content !== undefined && content !== null) {
     this.dom.inner.innerHTML = content;
   }
   else {
-    this.dom.inner.innerHTML = this.groupId;
+    this.dom.inner.innerHTML = this.groupId || ''; // groupId can be null
   }
 
   // update title