This message occours whenever Insync is installed or upgraded. It is not only written to the console, but also appears as a graphical yes/no dialog. It would be better if Insync would stay inside the console. Commands like dnf update should not spawn unexpected message boxes.
Upgrading : insync-1.3.4.36059-fc21.x86_64 25/58
which: no xmessage in (/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin)
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
OK
Insync installation has finished. You may now start it.
~$ sudo dpkg -i /tmp/insync_3.3.1.40899-focal_amd64.deb
Selecting previously unselected package insync.
(Reading database … 371402 files and directories currently installed.)
Preparing to unpack …/insync_3.3.1.40899-focal_amd64.deb …
Unpacking insync (3.3.1.40899-focal) …
Setting up insync (3.3.1.40899-focal) … > Insync installation has finished. You may now start it.
fs.inotify.max_user_watches = 1048576
Processing triggers for man-db (2.9.1-1) …
Processing triggers for hicolor-icon-theme (0.17-2) …
Processing triggers for shared-mime-info (1.15-1) …
Processing triggers for gnome-menus (3.36.0-1ubuntu1) …
Processing triggers for desktop-file-utils (0.24+linuxmint1) …
Processing triggers for mime-support (3.64ubuntu1) …
The way things are going in the future is toward systemd user service units that get installed in /usr/lib/systemd/user. The startup process insync start writes a PID file and points systemd to it (old fashioned way) or it just never forks and stays a “foreground daemon,” (the new systemd way). Both GNOME and KDE Plasma are shifting toward systemd service units for starting up the desktop, so Insync might want to jump on the bandwagon too. It has a number of advantages compared to using ~/.config/autostart. For example, you could probably reuse RPM packaging macros that handle typical system-wide systemd services to restart all insync.service instances when the package is upgraded.