If you want to use wmsystemtray with the Plasma 2 desktop environment because
that DE removed support for xembed applications, you will want to start it as fofollows:

wmsystemtray --non-wmaker --bgcolor white

And it is best to also configure a KWin window rule to have it without
decorations on all desktops:

# ----- 8< ----------------------------------------
[Application settings for wmsystemtray]
Description=Application settings for wmsystemtray
desktop=-1
desktoprule=2
noborder=true
noborderrule=2
skippager=true
skippagerrule=2
skipswitcher=true
skipswitcherrule=2
skiptaskbar=true
skiptaskbarrule=2
type=2
typerule=2
wmclass=wmsystemtray0 wmsystemtray
wmclasscomplete=true
wmclassmatch=1
# ----- 8< ----------------------------------------