attributeset_get_first(): set is NULL (gtk3 terminal widget)

[step-]

Reported in https://forum.puppylinux.com/viewtopic.php?p=39717#p39717

Gtkdialog [...] GTK3 and libvte support, so UExtract is using gtkdialog's <terminal> widget [...] However, <terminal> widget in Vanilla [...] does nothing on exit.

This error message is output along others: "attributeset_get_first(): set is NULL"

Reproduce by (needs GTK3 with libvte2.91):

MAIN_DIALOG='
<window>
    <vbox>
        <terminal argv0="/bin/sh"
            has-focus="true">
            <variable>vte0</variable>
            <input>echo "echo wait for it...; sleep 3 && exit"</input>
            <action signal="child-exited">exit:EXIT</action>
        </terminal>
    </vbox>
</window>' gtkdialog -c

Expected output: the vte terminal should exit and print EXIT="EXIT" Instead it does not exit.