mTts.synthesizeToFile("hola",null,new File("/mnt/sdcard/Download/sound.wav"),"record");

java.lang.NullPointerException: Attempt to invoke virtual method 'int android.speech.tts.TextToSpeech.synthesizeToFile(java.lang.CharSequence, android.os.Bundle, java.io.File, java.lang.String)' on a null object reference
                  at it.moondroid.chatbot.MainActivity$ResponseReceiver.onReceive(MainActivity.java:388)
                  at android.support.v4.content.LocalBroadcastManager.executePendingBroadcasts(LocalBroadcastManager.java:297)
                  at android.support.v4.content.LocalBroadcastManager.access$000(LocalBroadcastManager.java:46)
                  at android.support.v4.content.LocalBroadcastManager$1.handleMessage(LocalBroadcastManager.java:116)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:158)
                  at android.app.ActivityThread.main(ActivityThread.java:7225)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)