slapos_panel: Fixup if subscription request is a temp object

  If you access the listbox before the subscription request is indexed the link is broken on the listbox.

  patch provided by: Romain Courteaud.

master/bt5/slapos_panel/SkinTemplateItem/portal_skins/slapos_panel/InstanceTree_getCreateDirectDepositPaymentTransactionList.py

@@ -50,6 +50,16 @@ if outstanting_total_price > 0:
   if not context.Base_isExternalPaymentConfigured(currency_uid):
     raise ValueError("External Payment support is not configured")
 
+  if len(outstanding_amount_list) == 0:
+    # return payment_list
+    payment_list.append(context.asContext(
+      reference="Subscriptions pre-payment",
+      # Format by hand is not a good idea probably
+      stop_date=DateTime().strftime('%d/%m/%Y'),
+      resource_uid=currency_uid,
+      total_price=price,
+    ))
+  else:
     payment_list.append(outstanding_amount_list[0].asContext(
       reference="Subscriptions pre-payment",
       # Format by hand is not a good idea probably