The error message is referring to the High Availbility tab on the AP configuration. You likely have an IP address in the Primary/Secondary/Tertiary field without actually having a Controller Name. For High Availability, the IP is optional but the name is a requirement.
When you hit APPLY there is a config check on the GUI seeing this mismatch in configuration and is warning you that it can't apply the configuration because you are missing the WLC name. solution would be to remove the IP address from High Availability or to properly configure it with WLC names.
At least that is what I believe is happening.....
