Error sending commands by bluetooth

04-22 15:03:14.652 21137-21137/be.heinenbavo.adriunorobot E/AndroidRuntime: FATAL EXCEPTION: main
                                                                            Process: be.heinenbavo.adriunorobot, PID: 21137
                                                                            java.lang.NullPointerException
                                                                                at android.bluetooth.BluetoothSocket.write(BluetoothSocket.java:440)
                                                                                at android.bluetooth.BluetoothOutputStream.write(BluetoothOutputStream.java:85)
                                                                                at java.io.OutputStreamWriter.flushBytes(OutputStreamWriter.java:167)
                                                                                at java.io.OutputStreamWriter.flush(OutputStreamWriter.java:158)
                                                                                at java.io.BufferedWriter.flush(BufferedWriter.java:124)
                                                                                at be.heinenbavo.adriunorobot.MainActivity.sendDataToPairedDevice(MainActivity.java:119)
                                                                                at be.heinenbavo.adriunorobot.MainActivity.sendViaBluetooth(MainActivity.java:95)
                                                                                at be.heinenbavo.adriunorobot.MainActivity.access$000(MainActivity.java:41)
                                                                                at be.heinenbavo.adriunorobot.MainActivity$1.onClick(MainActivity.java:62)
                                                                                at android.view.View.performClick(View.java:4469)
                                                                                at android.view.View$PerformClick.run(View.java:18795)
                                                                                at android.os.Handler.handleCallback(Handler.java:808)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:103)
                                                                                at android.os.Looper.loop(Looper.java:193)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5299)
                                                                                at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                at java.lang.reflect.Method.invoke(Method.java:515)
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645)
                                                                                at dalvik.system.NativeStart.main(Native Method)