src/main/java/de/hitec/nhplus/nurse/LockedNurseController.java

@@ -113,6 +113,11 @@ public class LockedNurseController {
     @FXML
     public void handleMouseClick() {
         Nurse nurse = tableView.getSelectionModel().getSelectedItem();
+        if(nurse == null)
+        {
+            return;
+        }
+
         boolean canBeDeleted = nurse.calculateDeleteDate().isBefore(LocalDate.now());
         buttonDelete.setDisable(!canBeDeleted);
     }

src/main/resources/de/hitec/nhplus/treatment/database/Treatment.sql

@@ -10,5 +10,5 @@ CREATE TABLE treatment
     remark      TEXT    NOT NULL,
     isLocked    BOOLEAN NOT NULL DEFAULT false,
     FOREIGN KEY (patientId) REFERENCES patient (id) ON DELETE CASCADE,
-    FOREIGN KEY (nurseId) REFERENCES nurse (id) ON DELETE SET NULL
+    FOREIGN KEY (nurseId) REFERENCES nurse (id) ON DELETE CASCADE
 )