Im pretty sure i have found a bug with the treeview.
Here is a fiddle that illistrates the problem: http://jsbin.com/ocaraj/3/edit
To see the bug open up a console window and drag a node into a node that you have not loaded, ie. the nodes children have not been loaded. In the 'dragend' event (and possibly others) the e.sourceNode is null if you drop it on an unloaded node. But if the node has been loaded it works as normal, ie e.sourceNode is the dropped node.
Here is a fiddle that illistrates the problem: http://jsbin.com/ocaraj/3/edit
To see the bug open up a console window and drag a node into a node that you have not loaded, ie. the nodes children have not been loaded. In the 'dragend' event (and possibly others) the e.sourceNode is null if you drop it on an unloaded node. But if the node has been loaded it works as normal, ie e.sourceNode is the dropped node.