Are you sure wfMessage is not null?
You could also try putting a breakpoint or some logging in the microflow that is called to see if it is even reached when the error occurs, and if it does, if it fails somewhere in there. Are there any actions in there that could throw an error for example?
I suggest you debug the java action and check exactly when the error occurs and then you can fix it.
By the image it says about something is null when it shouldn’t be. Could be wfMessage or anything before that.
Try to get a starting point on knowing when exactly the exception occurs and then you can try to fix it.