Code (Javascript)

args.setCancel(true); 

Code (Javascript)

var node = args.getNode();
var container = args.getContainer();

var isAlreadyChild = container === node.getContainer(); 

Code (Javascript)

container.setAllowAddChildren(false); 

Code

function onContainerChildAdding(sender, args)
{
	var ctr = args.getContainer();

	// prevent third level nested containers

	// do not allow dropping container into an already nested container
	if (args.getNode().children && ctr.getContainer())
	{
		args.setCancel(true);
		args.setHandled(true);
	}

	// do not allow dropping parent of nested containers into a container
	if (args.getNode().children)
	{
		for (var child of args.getNode().children)
		{
			if (child.children)
			{
				args.setCancel(true);
				args.setHandled(true);
				break;
			}
		}
	}
}