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

product/ERP5Workflow/Document/Transition.py

@@ -93,14 +93,13 @@ class Transition(IdAsReferenceMixin("transition_", "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: