In this instance from step 4. It looks like this part of the checks related to the When clause.
And it seems to indicate that the step page is null.
But I'm a little puzzled as this looks like the same check which is done in step 1, yet this doesn't throw any exception there. Is it possible that the step page (local.interestPage in the calling SubmitModalFlowAction activity) could have been removed in the meantime perhaps?