Symptoms:

After a Bamboo build is complete, a user would be presented with an error :

 Notifier Build Complete Action custom build post complete action failed to run
(java.lang.NullPointerException)

Cause:

this is caused by a plugin called "notifier-bamboo-plugin" which is bundled in OnDemand only, and it's causing this issue.

Workaround:

Although this doesn't cause any harm, but if you want to get rid of the error message until the fix is released you can ask support to disable that function module in the plugin "notifier-bamboo-plugin".