Fix Reference widget when no model is selected, value is just text

issue3352
review1090002
(transplanted from 5b7b2c1ba521565cb9e052c33f980a28589a47de)

tryton/gui/window/view_form/view/form_gtk/reference.py

@@ -114,7 +114,12 @@ class Reference(Many2One):
             return
         self.wid_text.set_text('')
         self.wid_text.set_position(0)
-        self.field.set_client(self.record, (self.get_model(), (-1, '')))
+        model = self.get_model()
+        if model:
+            value = (model, (-1, ''))
+        else:
+            value = ('', '')
+        self.field.set_client(self.record, value)
 
     def set_value(self, record, field):
         if not self.get_model():