The solution presented here worked. Purge all virtualbox packages -- especially unity-scope-virtualbox -- and then install the latest .deb package from Oracle. Be sure to install from the command line. I don't trust Ubuntu's Software Manager any longer.

The solution presented here worked. Purge all virtualbox packages -- especially unity-scope-virtualbox -- and then install the latest .deb package from Oracle. These were the steps I took:

sudo apt-get purge virtualbox
sudo apt-get purge virtualbox-5.1
sudo apt-get purge virtualbox-5.2
sudo apt-get purge virtualbox-qt
sudo apt-get purge virtualbox-dkms
sudo apt-get purge unity-scope-virtualbox

sudo dpkg -i virtualbox-5.2_5.2.14-123301_Ubuntu_xenial_amd64.deb

Be sure to install from the command line. I don't trust Ubuntu's Software Manager any longer.