安卓 java。lang.NullPointerException:尝试从字段“long org”读取。opencv。果心小地毯空对象引用上的nativeObj

Question

我是openCV和安卓的新手。我正在尝试使用位图从资源中加载图像，并使用open CV处理图像。它给了我这个错误：

java.lang.NullPointerException: 
Attempt to read from field 'long org.opencv.core.Mat.nativeObj' on a null object reference

代码：

    Bitmap bm = BitmapFactory.decodeResource(getResources(),R.drawable.a1);
    ((ImageView)findViewById(R.id.imageView)).setImageBitmap(bm);
    Mat trainImage = new Mat(bm.getHeight(),bm.getWidth(),CvType.CV_32S);
    Utils.bitmapToMat(bm,trainImage);
    if(trainImage.empty()){
        Log.e(TAG,"Empty image");
    }
    else{
        Log.e(TAG,"Image Loaded");
    }
    Imgproc.cvtColor(trainImage, trainGray, Imgproc.COLOR_RGBA2GRAY);

以下是例外消息：

E/SVM: Image Loaded
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.pragyan.facefeatures, PID: 3308
java.lang.NullPointerException: Attempt to read from field 'long org.opencv.core.Mat.nativeObj' on a null object reference
  at org.opencv.imgproc.Imgproc.cvtColor(Imgproc.java:1724)
  at com.example.pragyan.facefeatures.ExtractFeature.trainSvm(ExtractFeature.java:242)
  at com.example.pragyan.facefeatures.ExtractFeature.onOptionsItemSelected(ExtractFeature.java:313)
  at 安卓.app.Activity.onMenuItemSelected(Activity.java:2885)
  at 安卓.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:403)
  at 安卓.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:189)
  at 安卓.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:100)
  at 安卓.support.v7.app.AppCompatDelegateImplV7.onMenuItemSelected(AppCompatDelegateImplV7.java:663)
  at 安卓.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:811)
  at 安卓.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:152)
  at 安卓.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:958)
  at 安卓.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:948)
  at 安卓.support.v7.view.menu.MenuPopupHelper.onItemClick(MenuPopupHelper.java:191)
  at 安卓.widget.AdapterView.performItemClick(AdapterView.java:305)
  at 安卓.widget.AbsListView.performItemClick(AbsListView.java:1146)
  at 安卓.widget.AbsListView$PerformClick.run(AbsListView.java:3053)
  at 安卓.widget.AbsListView$3.run(AbsListView.java:3860)
  at 安卓.os.Handler.handleCallback(Handler.java:739)
  at 安卓.os.Handler.dispatchMessage(Handler.java:95)
  at 安卓.os.Looper.loop(Looper.java:135)
  at 安卓.app.ActivityThread.main(ActivityThread.java:5254)
  at java.lang.reflect.Method.invoke(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:372)
  at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
  at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:698)

谁能告诉我怎么解决这个问题吗。图像已成功加载，但仍会显示NullPointerException