In case we have GESUriClip with wrong asset-id, ged-base-xml-formatter will never call _loading_done:

    convert -size 640x480 xc:white /tmp/asset-1.png
    cat <<EOF > p1.xges
<ges version='0.1'>
    <project properties='properties;' metadatas='metadatas;'>
        <ressources>
            <asset id="file:///tmp/asset-1.png" extractable-type-name="GESUriClip" properties="properties, supported-formats=(int)4;" metadatas="metadatas;" />
        </ressources>
        <timeline properties='properties, auto-transition=(boolean)true, snapping-distance=(guint64)0;' metadatas='metadatas, duration=(guint64)3000000000;'>
            <track caps="video/x-raw" track-type="4" track-id="0" properties="properties, async-handling=(boolean)false, message-forward=(boolean)false, caps=(string)video/x-raw, restriction-caps=(string)&quot;video/x-raw\,\ width\=\(int\)640\,\ height\=\(int\)480\,\ framerate\=\(fraction\)30/1&quot;;" metadatas="metadatas;" />
            <layer priority="0" properties="properties, auto-transition=(boolean)true;" metadatas="metadatas, volume=(float)1;">
                <clip id="1" asset-id="file:///tmp/asset-2.png" type-name="GESUriClip" layer-priority="0" track-types="4" start="0" duration="3000000000" inpoint="0" rate="0" properties="properties, name=(string)uriclip0, mute=(boolean)false, is-image=(boolean)true;"/>
            </layer>
        </timeline>
    </project>
</ges>
EOF

So, neither error nor "loaded" signal will be sent to GESPipeline's subscriber. Тext command will either never return or will fail with GstDiscoverer timeout:

    ges-launch-1.0 -l p1.xges -z p1.ogg

Obviously, we should show warning about unused asset and "unknown asset identifier" error.