Android : Attempt to invoke virtual method on a null object reference

Question

I am developing an android application that get a PDF file and extract its content and then convert this text to an audio file. I am using itext library to get parsed text and TTS Engine to save the audio file to specific directory through (synthesizeToFile) method. I think that every thing is OK but when i run the app i see an error and don't know what is that and how to solve it.

This is the error:

and here is my code:

Custom Adapter class:

public class CustomAdapter extends BaseAdapter {

Context c;
ArrayList<PDFDoc> pdfDocs;

private TextToSpeech mTts;
private Uri fileUri;
private Uri sendFileUri;

public CustomAdapter(Context c, ArrayList<PDFDoc> pdfDocs) {
    this.c = c;
    this.pdfDocs = pdfDocs;
}

@Override
public int getCount() {
    return pdfDocs.size();
}

@Override
public Object getItem(int i) {
    return pdfDocs.get(i);
}

@Override
public long getItemId(int i) {
    return i;
}

@Override
public View getView(int i, View view, ViewGroup viewGroup) {
    if (view == null) {
        //INFLATE CUSTOM LAYOUT
        view = LayoutInflater.from(c).inflate(R.layout.model, viewGroup, false);
    }

    final PDFDoc pdfDoc = (PDFDoc) this.getItem(i);

    TextView nameTxt = (TextView) view.findViewById(R.id.nameTxt);
    ImageView img = (ImageView) view.findViewById(R.id.pdfImage);

    //BIND DATA
    nameTxt.setText(pdfDoc.getName());
    img.setImageResource(R.drawable.pdf_icon);

    //VIEW ITEM CLICK
    view.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            try {
                String received = extractPDF(pdfDoc.getPath());
                sendFileUri=synthesizeToFile(received);
                Intent intent = new Intent(c,PDFMediaPlayer.class);
                intent.putExtra("PATH",pdfDoc.getPath());
                intent.setData(sendFileUri);
                c.startActivity(intent);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
    return view;
}

//Extract PDF
public String extractPDF(String path) throws IOException {

        String parsedText = "";
        PdfReader reader = new PdfReader(path);
        int n = reader.getNumberOfPages();
        for (int page = 0; page < n; page++) {
            parsedText = parsedText + PdfTextExtractor.getTextFromPage(reader, page + 1).trim() + "\n"; //Extracting the content from the different pages
        }
        reader.close();

    return parsedText;
}

public Uri synthesizeToFile (String received) throws IOException {

        HashMap<String, String> myHashRender = new HashMap();
        myHashRender.put(TextToSpeech.Engine.KEY_PARAM_UTTERANCE_ID, received);

        String exStoragePath = Environment.getExternalStorageDirectory().getAbsolutePath();
        File appTmpPath = new File(exStoragePath + "/sounds/");
        appTmpPath.mkdirs();
        //File file = new File();
        String tempFilename = "tts.wav";
        String tempDestFile = appTmpPath.getAbsolutePath() + "/" + tempFilename;

        mTts.synthesizeToFile(received, myHashRender, tempDestFile);

        File fileTTS = new File(tempDestFile);
        fileUri = Uri.fromFile(fileTTS);

    return Uri.parse(fileUri.getPath());
}

}

and here is my code for class PDFMediaPlayer:

public class PDFMediaPlayer extends AppCompatActivity {

private Button b1,b2,b3,b4;
private ImageView iv;

private double startTime = 0;
private double finalTime = 0;

private Handler myHandler = new Handler();;
private int forwardTime = 5000;
private int backwardTime = 5000;
private SeekBar seekbar;
private TextView tx1,tx2,tx3;

public static int oneTimeOnly = 0;

private MediaPlayer mediaPlayer;
private Uri mediaplayerUri;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_media_player);

    Intent i = this.getIntent();
    String path = i.getExtras().getString("PATH");
    mediaplayerUri = i.getData();
    File file = new File(path);

    b1 = (Button) findViewById(R.id.button);
    b2 = (Button) findViewById(R.id.button2);
    b3 = (Button) findViewById(R.id.button3);
    b4 = (Button) findViewById(R.id.button4);

    tx1 = (TextView) findViewById(R.id.textView2);
    tx2 = (TextView) findViewById(R.id.textView3);
    tx3 = (TextView) findViewById(R.id.textView4);
    tx3.setText(file.getName());

    mediaPlayer = MediaPlayer.create(this,mediaplayerUri);

    seekbar = (SeekBar) findViewById(R.id.seekBar);
    seekbar.setClickable(false);
    b2.setEnabled(false);
    b3.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(getApplicationContext(), "Playing sound", Toast.LENGTH_SHORT).show();
            mediaPlayer.start();

            finalTime = mediaPlayer.getDuration();
            startTime = mediaPlayer.getCurrentPosition();

            if (oneTimeOnly == 0) {
                seekbar.setMax((int) finalTime);
                oneTimeOnly = 1;
            }

            tx2.setText(String.format("%d min, %d sec",
                    TimeUnit.MILLISECONDS.toMinutes((long) finalTime),
                    TimeUnit.MILLISECONDS.toSeconds((long) finalTime) -
                            TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes((long)
                                    finalTime)))
            );

            tx1.setText(String.format("%d min, %d sec",
                    TimeUnit.MILLISECONDS.toMinutes((long) startTime),
                    TimeUnit.MILLISECONDS.toSeconds((long) startTime) -
                            TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes((long)
                                    startTime)))
            );

            seekbar.setProgress((int) startTime);
            myHandler.postDelayed(UpdateSongTime, 100);
            b2.setEnabled(true);
            b3.setEnabled(false);
        }
    });

    b2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(getApplicationContext(), "Pausing sound", Toast.LENGTH_SHORT).show();
            mediaPlayer.pause();
            b2.setEnabled(false);
            b3.setEnabled(true);
        }
    });

    b1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            int temp = (int) startTime;

            if ((temp + forwardTime) <= finalTime) {
                startTime = startTime + forwardTime;
                mediaPlayer.seekTo((int) startTime);
                Toast.makeText(getApplicationContext(), "You have Jumped forward 5 seconds", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(getApplicationContext(), "Cannot jump forward 5 seconds", Toast.LENGTH_SHORT).show();
            }
        }
    });

    b4.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            int temp = (int) startTime;

            if ((temp - backwardTime) > 0) {
                startTime = startTime - backwardTime;
                mediaPlayer.seekTo((int) startTime);
                Toast.makeText(getApplicationContext(), "You have Jumped backward 5 seconds", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(getApplicationContext(), "Cannot jump backward 5 seconds", Toast.LENGTH_SHORT).show();
            }
        }
    });
}
private Runnable UpdateSongTime = new Runnable() {
    public void run() {
        startTime = mediaPlayer.getCurrentPosition();
        tx1.setText(String.format("%d min, %d sec",
                TimeUnit.MILLISECONDS.toMinutes((long) startTime),
                TimeUnit.MILLISECONDS.toSeconds((long) startTime) -
                        TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.
                                toMinutes((long) startTime)))
        );
        seekbar.setProgress((int)startTime);
        myHandler.postDelayed(this, 100);
    }
};

}