Fixes disabled mute and sound buttons in a conference call.

Fixes NullPointerException causing problems with sound indicators in a conference call.

src/net/java/sip/communicator/impl/gui/main/call/CallPanel.java

@@ -360,6 +360,8 @@ public void stateChanged(ChangeEvent e)
         }
         else
         {
+            // These buttons are only added in the conference call. For the one
+            // to one call mute and sound buttons are in the call peer panel.
             localLevel.setEnabled(false);
             remoteLevel.setEnabled(false);
 
@@ -650,6 +652,8 @@ public void enableButtons()
         conferenceButton.setEnabled(true);
         holdButton.setEnabled(true);
         recordButton.setEnabled(true);
+        localLevel.setEnabled(true);
+        remoteLevel.setEnabled(true);
 
         if (!isLastConference)
         {

src/net/java/sip/communicator/impl/neomedia/device/AudioMixerMediaDevice.java

@@ -486,6 +486,10 @@ public AudioMixerMediaDeviceSession()
          */
         public void addLocalUserAudioLevelListener(SimpleAudioLevelListener l)
         {
+            // If the listener is null, we have nothing more to do here.
+            if (l == null)
+                return;
+
             synchronized(localUserAudioLevelListenersSyncRoot)
             {
                 //if this is the first listener that we are seeing then we also