See the code below

/// <summary>
       /// Add the specified name of a <see cref="IJobListener" /> to
the
       /// end of the <see cref="IJob" />'s list of listeners.
       /// </summary>
       public virtual void AddJobListener(string listenerName)
       {
                       if (!jobListeners.Add(listenerName))
                       {
                               throw new ArgumentException(string.Format
(CultureInfo.InvariantCulture, "Job listener '{0}' is already
registered for job detail: {1}", listenerName, FullName));
                       }
       }

       /// <summary>
       /// Remove the specified name of a <see cref="IJobListener" />
from
       /// the <see cref="IJob" />'s list of listeners.
       /// </summary>
       /// <returns>true if the given name was found in the list, and
removed</returns>
       public virtual bool RemoveJobListener(string listenerName)
       {
           for (int i = 0; i < jobListeners.Count; i++)
           {
               IJobListener listener = (IJobListener) jobListeners
[i];
               if (listener.Name == listenerName)
               {
                   jobListeners.RemoveAt(i);
                   return true;
               }
           }
           return false;
       }

You store listener names but try to get IJobListener iface on deletion