The error message will have the alarm actions name.
For example, for the alert below...
Diagnostic Server - Alarm Action Failure
Action for alarm 'Action/b42cf995-5cad-4dcc-82b9-796862c9fbac/WriteToEventLogOperation/Read Error' failed to execute: Action 'Hello World' failed : Internal error; null pointer
... the name of the alarm action is "Hello World."
Each alarm actions is stored in an xml and the name of the xml for the above example is "b42cf995-5cad-4dcc-82b9-796862c9fbac."
Go to Configure | Alarm Actions and look for the alarm actions name and modify the alarm action to make sure it is created properly (i.e. there is a condition) or delete the alarm action completely.
If modifying it or deleting the alarm action doesn't help, delete the corresponding XML file associated with the alarm action. Close Spotlight and go to the Diagnostic Server host and navigate to this path: C:\Program Files\Quest Software (or Dell)\Diagnostic Server. Do a search for the name of the XML file and delete the file.