Dynamic config for staves

Hi.

I would like to build a plugin to add valve fingering for brass instruments.
I want to offer a config mask to the user to configure the staves (slide position or valve fingering).

What I tried:
- in the onRun Method, build a config array property with the current staves.
- created a qml Dialog which uses the Repeater component to show the config the staffs

But if I start the plugin, it crashs with the Error Message "TypeError: Cannot read property 'length' of undefined". So it seems, that the qml components are rendered before the onRun method.

Does anyone know a solution for my problem or just hint?