Call to a member function get() on null
This is a known error that happens since Joomla 3.7 in DOCman 2.x. The Joomla 3.7.x release changed an internal API that broke backwards compatibility.
DOCman 3.x is updated already with a fix for this and is fully compatible with Joomla 3.7. We advise all our users to upgrade at their earliest convenience.
DOCman 2.1.6 also fixes this issue in case upgrading from DOCman 2.x to DOCman 3.x is not possible.
You can find our upgrade guides here: http://help.joomlatools.com/category/49-upgrading