No. I thought that might be the issue as I had a second config file in that directory. So I emptied the directory of all other config files.
I still get the same error message.
That got me wondering if something else was going on. So I enabled debug with the following
./logstash --log.level debug --path.settings /etc/logstash --path.data /usr/share/logstash/data/
That resulted in the following error appearing in /var/log/logstash/logstash-plain.log
[2018-06-08T09:53:53,193][DEBUG][logstash.instrument.periodicpoller.jvm] collector name {:name=>"ParNew"}
[2018-06-08T09:53:53,193][DEBUG][logstash.instrument.periodicpoller.jvm] collector name {:name=>"ConcurrentMarkSweep"}
[2018-06-08T09:53:53,194][DEBUG][logstash.instrument.periodicpoller.cgroup] Error, cannot retrieve cgroups information {:exception=>"NoMethodError", :message=>"undefined method `[]' for nil:NilClass"}
[2018-06-08T09:53:53,418][DEBUG][logstash.pipeline ] Pushing flush onto pipeline {:pipeline_id=>"main", :thread=>"#<Thread:0x30c60d4@/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:247 sleep>"}
Which has me wondering if this is related to ubuntu 18.04 which is what I am running on at this time.