Your version of Tor has bug which causes this messages (see #21056). Version 0.3.0.6 has a partial fix. If you install this version there are high chances that the messages will go away. Currently the developers are working on a full fix (see #17242).
So I would recommend to install a newer version. Please follow the advise at the instruction page for Debian and Ubuntu and add the following line to your sources.list
file:
deb http://deb.torproject.org/torproject.org tor-experimental-0.3.0.x-xenial main
After you updated the repositories you can choose to install the newest version.