Iterate over copy of fields items in record validate

The validation of the fields may trigger to fetch the definition of new fields.

issue11612
review417601003
(grafted from fd0f35037e459393853314fff8702f4656baf931)

--HG--
branch : 6.0

tryton/gui/window/view_form/model/record.py

@@ -391,7 +391,7 @@ class Record(SignalEvent):
         elif fields is None:
             self._check_load()
         res = True
-        for field_name, field in self.group.fields.items():
+        for field_name, field in list(self.group.fields.items()):
             if fields is not None and field_name not in fields:
                 continue
             if field.attrs.get('readonly'):