Hello,
Thank you for reporting. It is unlikely that this is an issue with WCMp. It is far more likely that this is an issue related to the gateway.
Please help me reproduce the issue step-by-step. Then I can look at why this happened.
You have traced the exact line where the error is thrown, but this might be not the root cause. Once I can reproduce the problem on my machine, I will be able to fix it, or at least tell you what went wrong.
with regards
Alex