Operator [] for typed QML list cannot be compiled if the returned object is used in some way. Attached example shows two cases where it fails in different ways.

                var item = buttons[i]
                if (item !== undefined) {
                    myButton = item

fails with potentially undefined, and:

                var item = buttons[i]
                if (item !== undefined) {
                    setButton(item, "button " + i)

fails with LoadElement for the buttons[i] line.