A nullpointer exception can be anything. Did you verify the operator that you are using? Is it equipped with an accessgroup? Is the accessgroup selected? Can you login manually with that specific user?
I added below code right after the ".baseUrl(testServerUrl)" and could confirm it worked correctly.
.userAgentHeader("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.149 Safari/537.36")