Actually we shouldn't use this method in Obj-Validate rule and we can use it in Edit Validate which i got confused earlier. So you can use this in edit validate rule, may be some design issue it can be if it is returning null. Could you share the screenshots of your configuration so that i can have a try.

By the way, i tried a sample use case to test the getStepPage() in my edit validate rule and it is working as expected as shown in attachment.