Exercises on substitution

Fatal error: Uncaught TypeError: Argument 1 passed to DbLayer::get_value_obj() must be of the type string, null given, called in /var/www/vhosts/ent.storkey.uk/html/inc/db/dbLayer.php on line 549 and defined in /var/www/vhosts/ent.storkey.uk/html/inc/db/dbLayer.php:393 Stack trace: #0 /var/www/vhosts/ent.storkey.uk/html/inc/db/dbLayer.php(549): DbLayer->get_value_obj(NULL, 'type') #1 /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php(3475): DbLayer->get_type(NULL) #2 /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php(718): exercisem->write() #3 /var/www/vhosts/ent.storkey.uk/html/inc/dynaPage.php(179): mainPanel->write('main-panel') #4 /var/www/vhosts/ent.storkey.uk/html/index.php(21): dynaPage->write() #5 {main} thrown in /var/www/vhosts/ent.storkey.uk/html/inc/db/dbLayer.php on line 393