add missing custom format_value for multi select widget (#25966)

combo/utils/forms.py

@@ -67,3 +67,7 @@ class MultiSortWidget(forms.SelectMultiple):
         if isinstance(data, MultiValueDict):
             return {'data': data.getlist(name)}
         return data.get(name, None)
+
+    def format_value(self, value):
+        value = value.get('data') or []
+        return super(MultiSortWidget, self).format_value(value)