Having null labels in the database throws the exception -

java.lang.NullPointerException
at java.lang.String.compareTo(String.java:1168)
at com.atlassian.confluence.labels.actions.RankedNameComparator.compare(RankedNameComparator.java:13)
at java.util.TreeMap.put(TreeMap.java:530)
at java.util.TreeSet.add(TreeSet.java:238)
at com.atlassian.confluence.labels.DefaultLabelManager.getRankedLabels(DefaultLabelManager.java:463)
at com.atlassian.confluence.labels.DefaultLabelManager.getMostPopularLabelsWithRanksInSpace(DefaultLabelManager.java:449)

Currently, there is no constraint in the Database which disallows null labels - we should check for nulls.

One of the known cause of this is due to Checklist Macros plugin's bug CHK-68.

Workaround

You can delete the null label from the database by:

1. Backup your database
2. Run this query to find the label id of the null label.

   select LABELID from LABEL where NAME is null;
   

3. Run this query to delete all the null label from the CONTENT_LABEL table. Replace the <LABEL_ID> with the labelid of the null label.

   DELETE FROM CONTENT_LABEL
   WHERE LABELID=<LABEL_ID>
   

4. Run this query to delete all the null label from the LABEL table.

   DELETE FROM LABEL
   WHERE LABELID=<LABEL_ID>
   

5. Restart Confluence