Don't fail while autodetecting invalid field in import window

on_row_expanded could be called on node without children by sig_autodetect, in
such case the returned child is an invalid GtkTreeIter. So it must ignore such
case to let sig_autodetect popup an error message.

issue4313
review7791002

tryton/gui/window/win_import.py

@@ -238,6 +238,9 @@ class WinImport(NoModal):
 
     def on_row_expanded(self, treeview, iter, path):
         child = self.model1.iter_children(iter)
+        # autodetect could call for node without children
+        if child is None:
+            return
         if self.model1.get_value(child, 0) is None:
             prefix_field = self.model1.get_value(iter, 1)
             name, model = self.fields[prefix_field]