Apache Nifi failed to start web server (shutting down) during DocGenerator.generate()

Apache-Nifi v-1.7.1 was running fine on AWS Centos machine but now on restart it thrown following error: (below content of /nifi-app.log)

            org.apache.nifi:nifi-standard-nar:1.7.1 || ./work/nar/extensions/nifi-standard-nar-1.7.1.nar-unpacked
    org.apache.nifi.processors.standard.FetchSFTP
            org.apache.nifi:nifi-standard-nar:1.7.1 || ./work/nar/extensions/nifi-standard-nar-1.7.1.nar-unpacked
    org.apache.nifi.processors.azure.eventhub.ConsumeAzureEventHub
            org.apache.nifi:nifi-azure-nar:1.7.1 || ./work/nar/extensions/nifi-azure-nar-1.7.1.nar-unpacked
    === End Processor types ===
2019-01-21 10:07:15,987 WARN [main] org.apache.nifi.web.server.JettyServer Failed to start web server... shutting down.
java.lang.NullPointerException: null
    at org.apache.nifi.documentation.DocGenerator.generate(DocGenerator.java:62)
    at org.apache.nifi.web.server.JettyServer.start(JettyServer.java:835)
    at org.apache.nifi.NiFi.<init>(NiFi.java:157)
    at org.apache.nifi.NiFi.<init>(NiFi.java:71)
    at org.apache.nifi.NiFi.main(NiFi.java:292)
2019-01-21 10:07:15,987 INFO [Thread-1] org.apache.nifi.NiFi Initiating shutdown of Jetty web server...
2019-01-21 10:07:15,988 INFO [Thread-1] org.apache.nifi.NiFi Jetty web server shutdown completed (nicely or otherwise).

sudo chown -R nifi:nifi /opt/nifi/
sudo service nifi start 

worked for me.