OpenGLRhi graphics API: main application window becomes blank if modal dialog is open

Steps to reproduce:

1. Download and build the attached project ModalsTest.zip
2. Run it
3. Click the button "Open". A modal dialog appears.

When the dialog becomes visible the main window becomes gray.

Expected: the main window should not change its appearance.

If you comment line 13 in file main.cpp:

{{ QQuickWindow::setGraphicsApi(QSGRendererInterface::OpenGLRhi);}}

then this bug will stop reproducing.

You can see this bug reproduction in the attached video.