Error when empty value

models/models.py

@@ -68,6 +68,7 @@ class Base(models.AbstractModel):
                             for sub in sub_res:
                                 each_res = ""
                                 for ressub in sub.values():
+                                    if ressub != None:
                                         if each_res:
                                             each_res += ', ' + ressub
                                         else: