显示广告后，应用程序关闭

Question

我有这个问题，应用程序可以正常运行一两次，然后关闭，android studio告诉我：

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.inkshared.gamer_hd_wallpaper, PID: 17686
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ProgressDialog.dismiss()' on a null object reference
        at com.inkshared.gamer_hd_wallpaper.ui.detail.WallpaperDetailFragment$DownloadFileAsync.onPostExecute(WallpaperDetailFragment.java:1453)
        at com.inkshared.gamer_hd_wallpaper.ui.detail.WallpaperDetailFragment$DownloadFileAsync.onPostExecute(WallpaperDetailFragment.java:1366)
        at android.os.AsyncTask.finish(AsyncTask.java:695)
        at android.os.AsyncTask.-wrap1(Unknown Source:0)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:712)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6626)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811)
D/FA: Logging event (FE): app_exception(_ae), Bundle[{ga_event_origin(_o)=clx, _r=1, ga_screen_class(_sc)=AdActivity, ga_screen_id(_si)=-254988858715173777, timestamp=1572676684488, fatal=1}]
W/CrashlyticsCore: Cannot send reports. Settings are unavailable.
V/FA: Recording user engagement, ms: 1623
D/FA: Logging event (FE): user_engagement(_e), Bundle[{ga_event_origin(_o)=auto, engagement_time_msec(_et)=1623, ga_screen_class(_sc)=AdActivity, ga_screen_id(_si)=-254988858715173777}]
Process 17686 terminated.

据我所知，我是一个初学者，他指出了这两行：

        @Override
        protected void onPostExecute(String unused) {
            mProgressDialog.dismiss();
            dismissDialog();
            progress.dismiss();

            if (mProgressDialog.isShowing()) {
                mProgressDialog.cancel();
                progress.dismiss();

            }

            if (isSuccess) {
                setDownloadCount(wallpaperViewModel.wallpapers.get(itemPosition).wallpaper_id);
                Toast.makeText(getContext(), getString(R.string.message__download_success), Toast.LENGTH_LONG).show();



            }

        }

据我所知，我是一个初学者，他指出了以下几行：

--------------第1453行----------------

    @Override
    protected void onPostExecute(String unused) {
        mProgressDialog.dismiss();
        dismissDialog();
        progress.dismiss();

        if (mProgressDialog.isShowing()) {
            mProgressDialog.cancel();
            progress.dismiss();

        }

        if (isSuccess) {
            setDownloadCount(wallpaperViewModel.wallpapers.get(itemPosition).wallpaper_id);
            Toast.makeText(getContext(), getString(R.string.message__download_success), Toast.LENGTH_LONG).show();



        }

    }

--------------------第1366行-----------------------

class DownloadFileAsync extends AsyncTask<String, String, String> {

    private boolean isSuccess = false;

    @Override
    protected void onPreExecute() {
        super.onPreExecute();

        showDialog();
    }

    @Override
    protected String doInBackground(String... aurl) {
        int count;

        try {
            URL url = new URL(aurl[0]);
            String fileNameAndExtension = aurl[1];
            HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setRequestProperty("Accept-Encoding", "identity");
            urlConnection.connect();

            InputStream input = new BufferedInputStream(url.openStream());
            String path = Environment.getExternalStorageDirectory().toString();
            File dir = new File( path,"/HD WALLPAPERS/");

            if (!dir.exists()) {
                boolean b = dir.mkdir();
                int i = 0;
            }
            File file = new File(dir, fileNameAndExtension);

            String imagePath =  file.getAbsolutePath();
            //scan the image so show up in album
            MediaScannerConnection.scanFile(getContext(),
                    new String[] { imagePath }, null,
                    new MediaScannerConnection.OnScanCompletedListener() {
                        public void onScanCompleted(String path, Uri uri) {

                        }
                    });


            try {
                OutputStream output = new FileOutputStream(file);

                byte data[] = new byte[1024];

                long total = 0;

                int lenghtOfFile = urlConnection.getContentLength();
                Log.d("ANDRO_ASYNC", "Lenght of file: " + lenghtOfFile);

                while ((count = input.read(data)) != -1) {
                    total += count;
                    publishProgress("" + (int) ((total * 100) / lenghtOfFile));
                    output.write(data, 0, count);
                }

                output.flush();
                output.close();
                input.close();

                isSuccess = true;
            } catch (Exception e) {
                isSuccess = false;
                Utils.psErrorLog("", e);
            }
        } catch (Exception e) {
            isSuccess = false;
            Utils.psErrorLog("", e);
        }
        return null;

    }

    protected void onProgressUpdate(String... progress) {
        Log.d("ANDRO_ASYNC", progress[0]);
        mProgressDialog.setProgress(Integer.parseInt(progress[0]));
    }



    @Override
    protected void onPostExecute(String unused) {
        mProgressDialog.dismiss();
        dismissDialog();
        progress.dismiss();

        if (mProgressDialog.isShowing()) {
            mProgressDialog.cancel();
            progress.dismiss();

        }

        if (isSuccess) {
            setDownloadCount(wallpaperViewModel.wallpapers.get(itemPosition).wallpaper_id);
            Toast.makeText(getContext(), getString(R.string.message__download_success), Toast.LENGTH_LONG).show();



        }

    }
}

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我希望能很好地使用照片，感谢您的帮助。