lastUpdate() issue function is not working correctly for issues without comments after upgrading to v6.9.0 - [indexing.LastUpdatedByIndexer]

Users upgrading to V6.9.0 are experiencing re-indexing problems due a bug in ScriptRunner.

LastUpdatedByIndexer – which adds information to the index and is needed to support the lastUpdated() JQL function – is failing for issues without comments. 

An example of the stack trace is attached to this ticket. 

Steps to reproduce

• Upgrade ScriptRunner to V6.9.0 from (awaiting previous version info)
• Perform a background re-index. 

Expected behaviour

Re-indexing works as usual. 

Actual behaviour

• Indexing is slowed down because an additional exception is thrown in LastUpdatedByIndexer and has to be handled. 
• Logs are getting filled with the stack trace for the exception thrown in LastUpdatedByIndexer occurring for issues without comments. 
• lastUpdate() issue function is not working correctly for issues without comments. 

2020-09-12 10:19:52,824-0700 JiraTaskExecutionThread-1 WARN user 616x4538x3 uery53 10.177.216.55,10.177.216.41 /secure/admin/jira/IndexReIndex!reindex.jspa [c.o.j.g.jql.indexing.LastUpdatedByIndexer] ScriptRunner last updated by indexing failed on KEY-123
com.atlassian.cache.CacheException: java.lang.NullPointerException
	at com.atlassian.cache.memory.DelegatingCache.get(DelegatingCache.java:212)
	at com.atlassian.cache.memory.DelegatingCache.get(DelegatingCache.java:163)
	at com.atlassian.cache.Cache$get$0.call(Unknown Source)
	at com.onresolve.jira.groovy.jql.LastCommentLookupService.getLastComment(LastCommentLookupService.groovy:69)
	at com.onresolve.jira.groovy.jql.LastCommentLookupService$getLastComment.call(Unknown Source)
	at com.onresolve.jira.groovy.jql.indexing.LastUpdatedByIndexer.addIndex(LastUpdatedByIndexer.groovy:51)

Workaround

There is no workaround. Adaptavist engineers are working to deploy an emergency fix as soon as possible.