NullPointerException when setting up indexing (lock directory permission problems)

People are reporting that indexing is failing in the setup wizard and when reindexing, with the error:

"Could not activate indexing: Error: java.lang.NullPointerException"

In the logs, one sees:

2005-10-07 16:35:49,128 ERROR [jira.issue.index.DefaultIndexManager] !!! issueWriter was null - bad !!!
2005-10-07 16:35:49,128 ERROR [web.action.setup.Setup] Error activating indexing: com.atlassian.jira.issue.index.IndexException: Error: java.lang.NullPointerException
com.atlassian.jira.issue.index.IndexException: Error: java.lang.NullPointerException
at com.atlassian.jira.issue.index.DefaultIndexManager.indexIssuesAndComments(DefaultIndexManager.java:298)
at com.atlassian.jira.issue.index.DefaultIndexManager.reIndexIssues(DefaultIndexManager.java:249)
at com.atlassian.jira.issue.index.DefaultIndexManager.reIndexAll(DefaultIndexManager.java:189)
at com.atlassian.jira.issue.index.DefaultIndexManager.activate(DefaultIndexManager.java:124)
at com.atlassian.jira.web.action.setup.Setup.doExecute(Setup.java:308)
at webwork.action.ActionSupport.execute(ActionSupport.java:153)
at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:47)
at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:132)
...
...
Caused by: java.lang.NullPointerException
at org.apache.lucene.store.FSDirectory.create(FSDirectory.java:149)
at org.apache.lucene.store.FSDirectory.<init>(FSDirectory.java:128)
at org.apache.lucene.store.FSDirectory.getDirectory(FSDirectory.java:102)
at org.apache.lucene.store.FSDirectory.getDirectory(FSDirectory.java:83)
at org.apache.lucene.index.IndexWriter.<init>(IndexWriter.java:173)
at com.atlassian.jira.util.LuceneUtils.getIndexWriter(LuceneUtils.java:46)
at com.atlassian.jira.issue.index.DefaultIndexManager.getIndexWriter(DefaultIndexManager.java:560)
at com.atlassian.jira.issue.index.DefaultIndexManager.indexIssuesAndComments(DefaultIndexManager.java:279)