java.lang.NullPointerException: Attempt to invoke virtual method

Nao consigo usar o metodo write(bytes[] );

public class MainActivity extends AppCompatActivity {
    private static final int REQUEST_ENABLE_BT = 123;
    private static final int REQUEST_MICROPHONE = 456;
    Bt_connect bt_connect;
    private static TextView subtitle;
    private boolean firstTime = true;
    public static Context context;
    private SpeechToText stt;
    private static TextView statusMessage;
    private Button mic_button;
    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        statusMessage = (TextView) findViewById(R.id.temp);
        subtitle = (TextView) findViewById(R.id.text_test);
        mic_button = (Button) findViewById(R.id.mic_button);
        PackageManager pm = getPackageManager();
        List<ResolveInfo> activities = pm.queryIntentActivities(
                new Intent(RecognizerIntent.ACTION_RECOGNIZE_SPEECH), 0);
        if (activities.size() == 0) {
            mic_button.setEnabled(false);
            showToast("Reconhecedor de voz nao encontrado");
        }
        BluetoothAdapter btAdapter = BluetoothAdapter.getDefaultAdapter();
        if (btAdapter == null) {
            Toast.makeText(MainActivity.this,"Sem suporte a BlueTooth",Toast.LENGTH_LONG).show();
        }
        if(!btAdapter.isEnabled()) {
            showToast("Ótimo! Hardware Bluetooth está funcionando :)");
            Intent turnon = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(turnon, REQUEST_ENABLE_BT);
            showToast("Bluetooth Ligado");
        }
        if (ContextCompat.checkSelfPermission(MainActivity.this,Manifest.permission.RECORD_AUDIO) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.RECORD_AUDIO},REQUEST_MICROPHONE);
        }


        context = getApplicationContext();
        if(isFirstTime()) {
            setSST();
        }

    }
/........\
public void onClickBt_connect(View view)throws InterruptedException{
        bt_connect = new Bt_connect("98:D3:34:90:F1:41");
        bt_connect.start();
    }

    public void MsgToArduino(String msg) {
            bt_connect.write("Teste\n".getBytes());

    }

Stack Trace

java.lang.NullPointerException: Attempt to invoke virtual method 'void skynet.home_control.Bt_connect.write(byte[])' on a null object reference
                                                                         at skynet.home_control.MainActivity.MsgToArduino(MainActivity.java:139)
                                                                         at skynet.home_control.SpeechToText.toMainActivity(SpeechToText.java:94)
                                                                         at skynet.home_control.SpeechToText.onPartialResults(SpeechToText.java:84)
                                                                         at android.speech.SpeechRecognizer$InternalListener$1.handleMessage(SpeechRecognizer.java:459)
                                                                         at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                         at android.os.Looper.loop(Looper.java:154)
                                                                         at android.app.ActivityThread.main(ActivityThread.java:6123)
                                                                         at java.lang.reflect.Method.invoke(Native Method)
                                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
                                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)