A doctor who acted as an adverse expert witness in a medical-negligence lawsuit cannot be sued over his testimony by the plaintiff in the case, a state appeals panel ruled Friday.Sole practitioner and plaintiff Philip W. Sandler alleged he suffered brain damage after stabbing himself 30 times while he was a patient at a west suburban hospital in Downers Grove.A Cook County jury awarded Sandler $4.2 million in May 2015, a record high for an inpatient suicide attempt in Illinois.During that trial, Dr. Jerry J. Sweet was …