BUGFIX #6089 Avoid javascript error when "Allow drag & drop reorderin…

…g" enabled, and attempt to drag a file from one folder to another is performed (from r111932)

git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/trunk@112945 467b73ca-7a2a-4603-9d3b-597d59a354a9

javascript/tree/tree.js

@@ -143,6 +143,8 @@ Tree.prototype = {
 	 * If 'before' is specified, then it will be inserted before that.
 	 */
 	appendTreeNode : function(child, before) {
+		if(!child) return;
+
 		// Remove from the old parent node - this will ensure that the classes of the old tree
 		// item are updated accordingly
 		if(child && child.parentTreeNode) {