It looks like that class was dropped from vala-panel in https://gitlab.com/vala-panel-project/vala-panel/commit/87493a6dfab9868f77b7b19b57fca40a06fe80af
Unfortunately the commit message doesn't give any clues to what if-anything it should be replaced with. If a proper fix cannot be found then one option would be to drop the vala-panel support from this package. The popcon figures for the vala-panel variant are the lowest of any of the variants. https://qa.debian.org/popcon.php?package=xfce4-sntray-plugin . I decided to go ahead and do that in raspbian for the moment to complete the transition there. A debdiff should appear soon at https://debdiffs.raspbian.org/main/x/xfce4-sntray-plugin . No intent to NMU in Debian. There does seem to be some upstream activity on xfce4-sntray-plugin at https://github.com/rilian-la-te/xfce4-sntray-plugin but I can't find anything relavent to this issue.
