Symptoms

The client noticed the following error in the event viewer about once every minute: 1 Object reference not set to an instance of an object.
 

Diagnoses

The client explained that they "appified" a SP2013 site and then later removed the K2 app from the site, because they wanted to change the name of the site.

After the site was renamed, it was "appified" again. After a long time, it was decided that there are not going to be any K2 apps on that site collection and it was removed again.
 

Resolution

We deleted the group provider from the K2].2HostServer].rGroupProvider] table and then deleted the settings for the site from the hK2].2Integration].nProcessSharePointSetting] table. After both were deleted and the K2Server restarted, all was well again.

Additional Information...
When you remove the K2 App from a site collection, we keep the K2 related configurations in the DB still. One might need to remove the K2 App for various reasons like when you want to make a template or copy an artifact from one SP farm to another.

If we removed the configuration information in the DB, your settings would have been lost and we had to re-create everything again. And if we recreate a smartobject, it will get created with a different GUID. If that smartobject was used anywhere on a view, form or in a workflow, everything that uses that will be broken and this why we keep the info in the DB.