When using Design Studio's way to handle signals in Connections {}, i.e. onFoo: {}, we get these errors when running:

QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... } 

If the signal handling in Connections {} is changed to that format, i.e. function onFoo() {}, the ui.qml file will not open in Design Studio anymore, except in text editor.