qmlglsink: crashes when created and destroyed in quick succession

qmlglsink crashes when created and destroyed in quick succession. Crash is very easy to reproduce, see snippet below:

import QtQuick 2.11
import org.freedesktop.gstreamer.GLVideoItem 1.0

Item {
    Loader {
        id: sinkLoader
        sourceComponent: Component { GstGLVideoItem {} }
    }

    Timer {
        interval: 1
        running: true
        repeat: true
        onTriggered: sinkLoader.active = !sinkLoader.active;
    }
}

Reproduced on:

• Ubuntu 18.04 (X11) with gst-plugins-good-1.14.5 and Qt 5.11.2
• Embedded Linux (Wayland) with gst-plugins-good-1.16.2 and Qt 5.11.2
• Embedded Linux (Wayland) with gst-plugins-good-master (5fb5abc8) and Qt 5.11.2

Analysis: It seems to be caused by a race condition where the render thread calls a method on the QtGLVideoItem instance that was previously destroyed in the main thread, here: https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/blob/master/ext/qt/qtitem.cc#L100 Also, less frequently, QtGLVideoItem::onSceneGraphInitialized is called when QQuickItem::window is null, also causing a crash.