First, your error is happening after SonarQube analysis. You should direct your question to the maintainers of the SonarGerrit plugin.
Second, 6.7.7 is past EOL. You should upgrade at your earliest convenience:
6.7.7 -> 7.9.4 -> 8.4.1 (this step is optional)
Ok, thanks, because Gerrit will need to be automatically paired with Jenkins + 1-1, so you can only use the lower version of Sonarqube