Fixed Nullpointer exception

app/src/main/java/fm/touhou/touhoufm/service/RadioService.java

@@ -280,13 +280,15 @@ public class RadioService extends Service implements AudioManager.OnAudioFocusCh
     private void stopMusic() {
         mPlaying = false;
 
-        mRingBuffer.abort();
+        if(mRingBuffer != null) {
+            mRingBuffer.abort();
 
+            mRingBuffer = null;
+        }
+        
         mAudioPlayer = null;
         mRtpReceiver = null;
 
-        mRingBuffer = null;
-
         runOnUiThread(new Runnable() {
             @Override
             public void run() {