membershipworks: Reject event invoice form if an invoice already exists

membershipworks/forms.py

@@ -22,6 +22,9 @@ class EventInvoiceForm(forms.Form):
         super().__init__(*args, **kwargs)
 
     def clean(self):
+        if hasattr(self.event, "invoice") is not None:
+            raise forms.ValidationError("An invoice was already created!")
+
         if self.event.total_due_to_instructor is None:
             raise forms.ValidationError(
                 "Event missing required information to generate invoice"