fix form generation if form contains proxy field with

empty form_id property

first slash in form_id of proxy field is not considered as empty
portal_skin of form

product/ERP5Form/ProxyField.py

@@ -435,7 +435,7 @@ class ProxyField(ZMIField):
     form_id = self.get_value('form_id')
     proxy_field = None
     form_id_with_skin_folder_name_flag = False
-    if '/' in form_id:
+    if form_id.find('/') > 0:
       # If a / is in the form_id, it means that skin_folder is explicitly
       # defined. If so, prevent acquisition to get the form.
       form_id_with_skin_folder_name_flag = True