Worklist.py: fix guard creation condiction to avoid guard conflict.

product/ERP5Workflow/Document/Worklist.py

@@ -89,14 +89,13 @@ class Worklist(IdAsReferenceMixin("worklist_", "prefix"), XMLObject):
           self.getGroupList() is None and\
           self.getExpression() is None and\
           self.guard is None:
-        return None
-      else:
+        return Guard().__of__(self)
+      elif self.guard is None:
         self.generateGuard()
       return self.guard
 
     def generateGuard(self):
-      if self.guard is None:
-        self.guard = Guard().__of__(self)
+      self.guard = Guard()
       if self.getRoleList() is not None:
         self.guard.roles = self.getRoleList()
       if self.getPermissionList() is not None: