How to fix CSF’s error Another app is currently holding the xtables lock

The error should be looking as below:

You have an unresolved error when starting csf:
Error: Error processing command for line [339] (6 times): [Another app is currently holding the xtables lock. Perhaps you want to use the -w option?], at line 339 in /usr/sbin/csf

You need to restart csf successfully to remove this warning, or delete /etc/csf/csf.errorRead the rest