VirtualKeyboard: Error reporting and engine customization

While experimenting with Qt/QML Virtual Keyboard module, I got bad uninformative (as always) error message:

QQmlApplicationEngine failed to load component
<Unknown File>:103:5: Cannot create input method engine

1. That's not an unknown file, that's my main.qml loaded by engine via "qrc:/main.qml" QUrl, and 103:5 is a place in it where I tried to instantiate InputEngine with some pre-defined properties.
2. Why can not create? For what reason? And what should I try instead?

Documentation of Virtual Keyboard severely lacks examples. I wanted numeric-only layout, which is reasonable to expect being available out-of-the-box. I found InputEngine QML type documentation and its inputMode which looks just like what I needed. I didn't find any way to obtain existing engine instance, so the best I could think of it to instantiate it myself, just like InputPanel nearby.

So, what's the correct way to use InputEngine, and how the error message can be improved?