I read on this forum that we should use IsBlank for text fields, which I assume holds true to formula text fields. I'm not sure however that holds true for flows as I don't see the option for IsBlank.
I have a flow that should exit the flow if this formula text field (It takes a URL and pulls out certain characters from the URL, think of it like pulling the 18 character SFDC ID out of a URL so you only have the 18 digit ID left) is null. I did that with a decision element that says: ID_Field__c IsNull $globalconstant.True which should exit the flow, but I instead get the unhandled fault error.
I'll post the error in case that helps. There are actually two errors, but my rep isn't at 10 yet so I just posted 1. As you can tell I'm pretty lost, but feel I may be close to getting this to work.