Call to a member function get() on null
This is a know 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.You can find our upgrade guides here: http://help.joomlatools.com/category/49-upgrading
In case upgrading from DOCman 2.x to DOCman 3.x is not possible please contact our support team for further assistance.