PHP error happened

Technical information
Error type2
Error descriptionTrying to access array offset on value of type null
URLscando.ist.berkeley.edu/dfl2db/person_view.php?action=view&id=940
Error file/srv/www/html/dfl2db/person_view.php
Error line209
SQL querySELECT * FROM informants WHERE 1=1 AND informants.informant_id=""