Opengsn forwarder verify signature error

Hi,

I am using openGSN forwarder for gasless transactions and trying to verify the signature. Am calling the verify method of the forwarder but getting the exception at this line

I am using the kovan forwarder provided the link below. I have tried to add new domain separator with the abi but throws error that method registerDomainSeparator is not present. What domain separator should be used?

registerDomainSeparator is used to register the domain name, version and verifying contract (=the forwarder itself)
With GSN, we use a utility method (here) to register our domain name “GSN Relayed Transaction” and version “2”

How did you try to call that method? the simplest way is to call it manually through etherscan (in the link above).
The emitted event will contain the domainSeparator value.
Note that when you call signTypedData to pop up the user a sign dialog, you pass the same values, and metamask uses them to display in the signing dialog, and then it encodes them into the same domainSeparator value to calculate the hash and signature.