Blank artifacts when using qml objects

Preliminary steps:
- Open MuseScore with the Promenade example.
- Plugins -> Plugin Manager.
- Check "load" for all available plugins.
- Close and restart MuseScore.

Three bugs:
-1-
- View -> Synthesizer -> Master Effects.
- Change "Effect A" from "Zita1" to "No Effect".

- Result: blank rectangle, see attached picture.

-2-
- Plugins -> scorelist.
- Result: blank gray window.

-3-
- Plugins -> ScoreView.
- Result: blank white window.

Notes on -1-:
A debug build shows the following in the command prompt:
Warning: qrc:/noeffect/noeffect.qml:1:1: plugin cannot be loaded for module "QtQuick": Namespace 'QtQuick' has already been used for type registration
Debug: no root object for NoEffect
but only when the other plugins have been registered and added to the menu.
Under Mac, even without loading the plugins both the effects show blank white rectangles.

Windows 8, Nightly build 654b329