Fix form validation of cancellation form

src/pretix/control/views/orders.py

@@ -1178,7 +1178,7 @@ class OrderTransition(OrderView):
             }
         )
 
-    def post(self, *args, **kwargs):
+    def post(self, request, *args, **kwargs):
         to = self.request.POST.get('status', '')
         if self.order.status in (Order.STATUS_PENDING, Order.STATUS_EXPIRED) and to == 'p' and self.mark_paid_form.is_valid():
             ps = self.mark_paid_form.cleaned_data['amount']
@@ -1267,7 +1267,8 @@ class OrderTransition(OrderView):
                                                  'confirmation mail.'))
             else:
                 messages.success(self.request, _('The payment has been created successfully.'))
-        elif self.order.cancel_allowed() and to == 'c' and self.mark_canceled_form.is_valid():
+        elif self.order.cancel_allowed() and to == 'c':
+            if self.mark_canceled_form.is_valid():
                 try:
                     cancel_order(self.order.pk, user=self.request.user,
                                  email_comment=self.mark_canceled_form.cleaned_data['comment'],
@@ -1293,12 +1294,14 @@ class OrderTransition(OrderView):
                             ))
 
                     messages.success(self.request, _('The order has been canceled.'))
+            else:
+                return self.get(self.request, *args, **kwargs)
         elif self.order.status == Order.STATUS_PENDING and to == 'e':
             mark_order_expired(self.order, user=self.request.user)
             messages.success(self.request, _('The order has been marked as expired.'))
         return redirect(self.get_order_url())
 
-    def get(self, *args, **kwargs):
+    def get(self, request, *args, **kwargs):
         to = self.request.GET.get('status', '')
         if self.order.status in (Order.STATUS_PENDING, Order.STATUS_EXPIRED) and to == 'p':
             return render(self.request, 'pretixcontrol/order/pay.html', {