uibuilder offers middleware functions for ExpressJS and Socket.IO. These sit in uibuilder's .config folder and you should find several *.js-template files there which give examples of use. uibuilder checks to make sure that, if there is, lets say, a sioUse.js file, that it contains the correct function style. If not, it will warn you. If you don't have that file there, it shouldn't give a warning but perhaps it is. A small bug that I will make a note of and squash when I get a chance.
The good news is that it does not impact your use of uibuilder. The middleware is ignored and processing continues.
If you do have that file present, it means that it does not contain a valid function and you would need to fix that in order to use the middleware.