Intenté usar los siguientes códigos para

Dibujar sobre lienzoGuarda el lienzo en la imagen

Problema: cuando intento guardar la imagen, muestra un error de puntero nulo y no se guarda nada.

Por favor, ayúdeme a encontrar el problema con el código o sugiérame una alternativa, que haga exactamente lo mismo. Gracias por adelantado.

Código para dibujar sobre lienzo:

package com.example.draw2;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class MyDrawView extends View {
    private Bitmap  mBitmap;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;
    private Paint   mPaint;

    public MyDrawView(Context c, AttributeSet attrs) {
        super(c, attrs);

        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);

        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setColor(0xFF000000);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(9);
    }


    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

        canvas.drawPath(mPath, mPaint);


    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }
    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;
        }
    }
    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }

    public Bitmap getBitmap()
    {
    return mBitmap;
    }



    public void clear(){
        mBitmap.eraseColor(Color.GREEN);
        invalidate();
        System.gc();

    }

}

El segundo código para guardar el lienzo como imagen está en la actividad principal.

Probé algunas cosas y se comenta alguna parte del código.

Como soy principiante, agradezco cualquier consejo.

Segundo código de actividad principal:

package com.example.draw2;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import android.app.Activity;
import android.graphics.Bitmap;
import android.os.Bundle;
import android.os.Environment;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;


public class MainActivity extends Activity 
{
    MyDrawView myDrawView;


    @Override
    protected void onCreate(Bundle savedInstanceState) 
    {

        super.onCreate(savedInstanceState);
        myDrawView = new MyDrawView(this, null);
        setContentView(R.layout.activity_main);

        Button button1 = (Button)findViewById(R.id.button1);    
        button1.setOnClickListener(new View.OnClickListener() 
        {
            public void onClick(View v)
            {       
                //View content = myDrawView;

                //System.out.println(content);


                //Bitmap bitmap = content.getDrawingCache();

                File folder = new File(Environment.getExternalStorageDirectory().toString());
                 boolean success = false;
                 if (!folder.exists()) 
                 {
                     success = folder.mkdirs();
                 }

                 System.out.println(success+"folder");

                 File file = new File(Environment.getExternalStorageDirectory().toString() + "/sample.JPEG");

             if ( !file.exists() )
             {
                   try {
                    success = file.createNewFile();
                } catch (IOException e) {
                    e.printStackTrace();
                }
             }

             System.out.println(success+"file");



             FileOutputStream ostream = null;
                try
                {
                ostream = new FileOutputStream(file);

                System.out.println(ostream);

                Bitmap save = myDrawView.getBitmap();
                if(save == null) {
                    System.out.println("NULL bitmap save\n");
                }
                save.compress(Bitmap.CompressFormat.PNG, 100, ostream);
                //bitmap.compress(Bitmap.CompressFormat.PNG, 100, ostream);
                   ostream.flush();
                    ostream.close();
                }catch (NullPointerException e) 
                {
                    e.printStackTrace();
                    Toast.makeText(getApplicationContext(), "Null error", Toast.LENGTH_SHORT).show();
                }

                catch (FileNotFoundException e) 
                {
                    e.printStackTrace();
                    Toast.makeText(getApplicationContext(), "File error", Toast.LENGTH_SHORT).show();
                }

                catch (IOException e) 
                {
                    e.printStackTrace();
                    Toast.makeText(getApplicationContext(), "IO error", Toast.LENGTH_SHORT).show();
                }

            }
        });

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
}