I am also finding it hard to test & debug app crashes in the AppBuilder with NativeScript. I use both the in-browser and windows client with the help of a iOS companion app.
My app kept crashing because of a simple syntax error (forgot one letter on a property). After 3 hours of trying to understand how to debug a NativeScript app (where do the errors pop-up?), I gave up and started to read the code line by line letter by letter, and foud the error.
Not even console.log() helped since nothing was logged. The main Log window logged only when a file was saved or moved. DeviceLog allways empty, iPhone logg show only some data while loading the app, no events or console.log. Same goes for the Output window in the in-browser client.