Failed using pdfRenderer on my device : 'void dalvik.system.CloseGuard.warnIfOpen()'

Question

I tried to display PDF with this example from YouTube : "PDFRenderer APIs to display PDF documents"

It works with an emulator (even if the page is not displayed on the entire screen, it's another problem), but when I try on my own device, I've this error :

java.lang.NullPointerException: Attempt to invoke virtual method 'void dalvik.system.CloseGuard.warnIfOpen()' on a null object reference

Here is my MainActiviy.java:

public class MainActivity extends Activity {
    private ImageView imageView;
    private int currentPage = 0;
    private Button next, previous;

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

        next = (Button) findViewById(R.id.next);
        previous = (Button) findViewById(R.id.previous);

        render();


        next.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                currentPage++;
                render();
            }
        });

        previous.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                currentPage--;
                render();
            }
        });

    }

    private void render() {
        try{
            imageView = (ImageView)findViewById(R.id.image);
            int REQ_WIDTH = imageView.getWidth();
            int REQ_HEIGHT = imageView.getHeight();

            Bitmap bitmap = Bitmap.createBitmap(REQ_WIDTH, REQ_HEIGHT, Bitmap.Config.ARGB_4444);
            File file = new File("/sdcard/Download/linuxfun.pdf");
            //File file = new File("/storage/emulated/0/Download/SE.pdf");
            PdfRenderer renderer = new PdfRenderer(ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY));

            if(currentPage < 0) {
                currentPage = 0;
            } else if(currentPage > renderer.getPageCount()) {
                currentPage = renderer.getPageCount() - 1;
            }

            Matrix m = imageView.getImageMatrix();
            Rect rect = new Rect(0, 0, REQ_WIDTH, REQ_HEIGHT);
            renderer.openPage(currentPage).render(bitmap, rect, m, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY);
            imageView.setImageMatrix(m);
            imageView.setImageBitmap(bitmap);
            imageView.invalidate();

        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}

In my AndroidManifest.xml , I put :

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

I thought it was the cause of my problem, but not.

Thank you for your help and forgive me for my imperfect english.