If you set up persistence with restoreOnStartup those Item will be initialized to their previous value when OH starts up instead of Undefined which is why that rule is throwing an error.
Not entirely certain as to why you are seeing the error now and didn’t before, but I believe using persistence in this case is the best answer.
See the following for details:
The New Rule Engine is experimental so there could be some other things going on as well. I still think persistence will address your issue.
