Adds frequency change event listener handler to LSM decoder.

src/module/decode/p25/P25_LSMDecoder.java

@@ -121,7 +121,14 @@ public class P25_LSMDecoder extends P25Decoder implements IComplexBufferListener
     @Override
     public Listener<FrequencyChangeEvent> getFrequencyChangeListener()
     {
-        return null;
+        return new Listener<FrequencyChangeEvent>()
+        {
+            @Override
+            public void receive(FrequencyChangeEvent frequencyChangeEvent)
+            {
+                //Ignored
+            }
+        };
     }
 
     @Override

src/sample/Broadcaster.java

@@ -58,6 +58,11 @@ public class Broadcaster<T> implements Listener<T>
 	
 	public void addListener( Listener<T> listener )
 	{
+	    if(listener == null)
+        {
+            throw new IllegalArgumentException("Listener cannot be null");
+        }
+
 		mListeners.add( listener );
 	}