Fix NPE if removing non-existant listener type

Fixes issue 4039

Change-Id: Ie5ff2edc27c062cf6ff671a8cc88bcd2c80da075
Reviewed-on: http://gerrit.dmdirc.com/1184
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Gregory Holmes <greg@dmdirc.com>

src/com/dmdirc/util/ListenerList.java

     /**
      * Adds a new listener of the specified type to this listener list.
      * 
+     * @param <T> The type of listener to be added
      * @param listenerType The type of listener to be added
      * @param listener The listener to be added
      */
     /**
      * Removes the specified listener from the list of listeners for the
      * specified type.
-     * 
+     *
+     * @param <T> The type of listener to be removed
      * @param listenerType The type that the listener should be removed from
      * @param listener The listener to be removed
      */
     public <T> void remove(final Class<T> listenerType, final T listener) {
-        listeners.get(listenerType).remove(listener);
+        get(listenerType).remove(listener);
     }
     
     /**
      * @param listener The listener to be removed
      */
     public void remove(final String listenerType, final Object listener) {
-        listeners.get(listenerType).remove(listener);
+        get(listenerType).remove(listener);
     }
     
     /**
      * Retrieves the list of listeners for the specified type.
-     * 
+     *
+     * @param <T> The type of listener to be retrieved
      * @param listenerType The type of listener that's being retrieved
      * @return A list of listeners for the specified type
      */