gtksink is not rendering my video (black screen)

[pisymbol .]

If I do the following on the command line:

gst-launch-1.0  filesrc
location="/mnt/storage/2cam/20190608_050606Z_00000000.mkv" ! matroskademux
name=demux demux.video_0 ! h264parse ! avdec_h264 ! videoconvert ! gtksink

I see the video in the Cairo window.

However, if I do the exact same thing in Python:

on_realize():
 53        self.pipeline = Gst.Pipeline()
 54        factory = self.pipeline.get_factory()
 55        gtksink = factory.make('gtksink')
 56        self.pipeline.add(gtksink)
 57        self.pipeline.add(self.bin) # self.bin =
Gst.parse_bin_from_dsecription(pipeline above)
 58        self.bin.link(gtksink)
 59        self.pack_start(gtksink.props.widget, True, True, 0)
 60        self.pipeline.set_state(Gst.State.PLAYING)
 61        gtksink.props.widget.show_all()

It pops open a dialog with a black screen.

HOWEVER, if I simply make my pipline above just 'videotestsrc' it plays the
test stream without issue in my window.

Why does 'videotestsrc' work but my custom pipeline above does not?

-aps
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