Task Searchlist Error - Value cannot be null

On the tasks tab of a Matter I clicked a global action that added some milestone tasks, followed by a CurrentFile.Update and CurrentPlan.Refresh.
When I clicked the tasks that were added I got an error (not supplied). I checked the searchlist and there is no ItemHover event in the code, so I assume it's got something to do with the ADDFMTASKSFIL addin?
Is there a reason this error occurred/can we stop it happening? I've replicated the action but cannot replicate the error. I've also seen the error before on a users machine.