raise exception if person responsible for the paper form is missing

contracts/models.py

@@ -746,6 +746,22 @@ class Contract(NameStrMixin, models.Model):
                 }
             )
 
+        if (
+            self.paper_form_state not in (
+                self.PaperFormStates.STORED,
+                self.PaperFormStates.SHREDDED,
+                self.PaperFormStates.LOST
+            )
+            and not self.paper_form_person_responsible
+        ):
+            raise ValidationError(
+                {
+                    "paper_form_person_responsible": (
+                        "Musí být vyplněno, pokud smlouva není uložená / skartovaná / ztracená."
+                    )
+                }
+            )
+
         if (
             self.valid_start_date
             and self.valid_end_date