In case you are using Autofac you'll need to registed the UI-O-Matic tree controller
builder.RegisterApiControllers(typeof(UIOMatic.Web.Controllers.UIOMaticTreeController).Assembly);
You'll get the error
"Message": "An error has occurred.","ExceptionMessage": "Object reference not set to an instance of an object.","ExceptionType": "System.NullReferenceException","StackTrace": " at Umbraco.Web.Trees.ApplicationTreeExtensions.TryLoadFromControllerTree ...
For an overview of issues or to report new ones please go to https://github.com/TimGeyssens/UIOMatic/issues​