I've not checked on the 159 / Brera / Spider exactly how this is done, but I've experienced the issue previously. There will likely be somewhere a very basic switch, which consists of a little [black] plastic rod on a spring that is pushed in when the door shuts and pokes out when door opens. it is somewhere in the door jam, often along the back edge or close to the back of the door along the B pillar. Take a look and see if there is one.
If there is, simple test is to take it off and swap it with the other side and see if the error follows the sensor, so if you swap from driver to passenger see if it says that door is open rather than the other.
if it does follow, problem solved you need a new sensor, grab one from a breaker for pennies. otherwise you know you've got a wiring issue with the door sensor.