Custom Gst.Element in GTK3 (not working)

from gi.repository import GLib
from gi.repository import GObject
from gi.repository import Gst
from gi.repository import Gtk

# Needed for window.get_xid(), xvimagesink.set_window_handle(), respectively:
from gi.repository import GdkX11, GstVideo

GObject.threads_init()
Gst.init(None)



class TestFilter(Gst.Element):

    __gstmetadata__ = (
        "TestFilter plugin",
        "test_filter.py",
        "Description",
        "Contact"
    )

    # src pad template: used to forward buffers
    _srctemplate = Gst.PadTemplate.new('video_src',
        Gst.PadDirection.SRC,
        Gst.PadPresence.ALWAYS,
        Gst.caps_from_string('video/x-raw, format=(string){ YV12, AYUV, YUY2, UYVY }')
    )

    # sink pad template: used to receive buffers
    _sinktemplate = Gst.PadTemplate.new('video_sink',
        Gst.PadDirection.SINK,
        Gst.PadPresence.ALWAYS,
        Gst.caps_from_string('video/x-raw, format=(string){ YV12, AYUV, YUY2, UYVY }')
    )

    # register pad templates
    __gsttemplates__ = (_srctemplate, _sinktemplate)

    def __init__(self):
        Gst.Element.__init__(self)

        # src pad
        self.srcpad = Gst.Pad.new_from_template(self._srctemplate, 'video_src')

        # sink pad
        self.sinkpad = Gst.Pad.new_from_template(self._sinktemplate, 'video_sink')
        self.sinkpad.set_chain_function_full(self._sink_chain)

        # add pads
        self.add_pad(self.srcpad)
        self.add_pad(self.sinkpad)

    def _sink_chain(self, pad, parent, buf):
        # ... do analyzing stuff ...
        # forward buffer
        return self.srcpad.push(buf)



if __name__=='__main__':
    def on_message(bus, message):
        if message.type == Gst.MessageType.EOS:
            player.set_state(Gst.State.NULL)
        elif message.type == Gst.MessageType.ERROR:
            player.set_state(Gst.State.NULL)
            (err, debug) = message.parse_error()
            print "Error: %s" % err, debug

    def on_sync_message(bus, message):
        if message.get_structure().get_name() == "prepare-window-handle":
            win_id = videowidget.get_property('window').get_xid()
            imagesink = message.src
            imagesink.set_property("force-aspect-ratio", True)
            imagesink.set_window_handle(win_id)

    def on_delete(*args):
        player.set_state(Gst.State.NULL)
        Gtk.main_quit()

    win = Gtk.Window()
    win.connect("delete_event", on_delete)
    videowidget = Gtk.DrawingArea()
    videowidget.set_size_request(400, 250)
    vbox = Gtk.VBox(False, 0)
    vbox.pack_start(videowidget, True, True, 0)
    win.add(vbox)
    win.show_all()

    # Setup GStreamer
    player = Gst.ElementFactory.make("playbin", "MultimediaPlayer")
    bus = player.get_bus()
    bus.add_signal_watch()
    bus.enable_sync_message_emission()
    bus.connect("message", on_message)
    bus.connect("sync-message::element", on_sync_message)

    # create a Bin containing my own TestFilter, textoverlay and autovideosink
    testfilter = TestFilter()
    textoverlay = Gst.ElementFactory.make('textoverlay', 'textoverlay')
    autovideosink = Gst.ElementFactory.make('autovideosink')
    video_bin = Gst.Bin.new('video_bin')
    video_bin.add(testfilter)
    video_bin.add(textoverlay)
    video_bin.add(autovideosink)
    sink_pad = Gst.GhostPad.new('sink', testfilter.get_static_pad('video_sink'))
    video_bin.add_pad(sink_pad)
    testfilter.link(textoverlay)
    textoverlay.link(autovideosink)
    player.set_property('video-sink', video_bin)

    textoverlay.set_property('text', 'ABC')

    player.set_property('uri', "file:///.../somefile.avi")
    player.set_state(Gst.State.PLAYING)

    Gtk.main()