Hi,
Have you tried calling setupGraphicsBackend() on your QQuickVTKRenderWindow instance as suggested by the message? According to its documentation:
This method sets the graphics API to OpenGLRhi and sets up the surface format for intermixed VTK and QtQuick rendering. Use this method before instantiating a QApplication/QGuiApplication in a QtQuick/QML app with a VTK render view like QQuickVTKRenderItem.
(emphasis added)
Source: VTK: QQuickVTKRenderWindow Class Reference
regards,
Paulo