I can easily reproduce the message e.g. by setting
GTK3_MODULE=xapp-gtk3-module while not having libxapp-gtk3-module
installed. The package's configuration file
/etc/X11/Xsession.d/80xapp-gtk3-module also sets that for the Xsession.
However... i) I can influence the message independent of
gir1.2-xapp-1.0. If it's referenced in $GTK3_MODULE and
libxapp-gtk3-module is installed, the message does not show up even if
gir1.2-xapp-1.0 is not installed. ii) It's harmless. blueman-manager
does not terminate in any way.
Actually, what return code do you get? In case of a crash, could you get
a backtrace? Otherwise, try blueman-manager --loglevel debug to find why
it exits.
