AndroidでAsyncTaskクラスとGlideライブラリを使ってDialogを表示する

AndroidでAsyncTaskクラスとGlideライブラリを使って，URLの画像を含めたDialigを生成しようとしています．
AsyncTaskクラスで onPostExecute() の中で，Dialogを生成する関数を呼び出しています．
また，Dialogを生成するクラスは genAlertDialog() 関数です．引数はString型のurl文字列で，この中でレイアウトを呼び出して，Glideを使用し，url画像を読み込んで，Dialog生成を試みています．
これに対して出てくるエラーは，

09-01 08:08:42.616 11664-11664/com.example.fmsuvm.sharetter E/AndroidRuntime: FATAL EXCEPTION: main
                                                                              Process: com.example.fmsuvm.sharetter, PID: 11664
                                                                              java.lang.IllegalArgumentException: You must pass in a non null View
                                                                                  at com.bumptech.glide.GenericRequestBuilder.into(GenericRequestBuilder.java:678)
                                                                                  at com.bumptech.glide.DrawableRequestBuilder.into(DrawableRequestBuilder.java:448)
                                                                                  at com.example.fmsuvm.sharetter.MainActivity.takenAlertDialog(MainActivity.java:520)
                                                                                  at com.example.fmsuvm.sharetter.MainActivity$AsyncPicLoad.onPostExecute(MainActivity.java:456)
                                                                                  at com.example.fmsuvm.sharetter.MainActivity$AsyncPicLoad.onPostExecute(MainActivity.java:402)
                                                                                  at android.os.AsyncTask.finish(AsyncTask.java:636)
                                                                                  at android.os.AsyncTask.access$500(AsyncTask.java:177)
                                                                                  at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:653)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:111)
                                                                                  at android.os.Looper.loop(Looper.java:194)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:5637)
                                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                                  at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)

です．
以下にコードを載せます.

java
1/* MainActivity.java */
2
3/* 前略 */
4
5@Override
6protected void onCreate(Bundle savedInstanceState) {
7        super.onCreate(savedInstanceState);
8        setContentView(R.layout.activity_main);
9        AsyncPicLoad mAsyncPicLoad = new AsyncPicLoad(getApplicationContext());
10        mAsyncPicLoad.execute(new String[] {"url sample"});
11}
12
13
14
15/* 中略 */
16
17private class AsyncPicLoad extends AsyncTask<String, Void, String>{
18
19        Context context;
20        String imageUrl;
21        public AsyncPicLoad(Context context){
22            this.context = context;
23        }
24
25        protected void onPreExecute(){
26            super.onPreExecute();
27            builder = new AlertDialog.Builder(new ContextThemeWrapper(MainActivity.this, R.style.myDialog));
28        }
29
30        @Override
31        protected String doInBackground(String... params) {
32            String result = null;
33            imageUrl = params[0];
34            return imageUrl;
35        }
36
37        @Override
38        protected void onPostExecute(String result){
39            super.onPostExecute(result);
40       
41            
42            takenAlertDialog(imageUrl);
43        }
44
45    }
46
47
48/* 中略 */
49
50public void takenAlertDialog(final String imageUrl){
51        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
52        builder.setTitle("test showing");
53
54        LayoutInflater factory = LayoutInflater.from(MainActivity.this);
55
56        final View view = factory.inflate(R.layout.dialog_layout, (ViewGroup)findViewById(R.id.root_layout));
57        ImageView dialog_urlImage = (ImageView)findViewById(R.id.showing_image);
58        Glide.with(this)
59                .load(imageUrl)
60                .crossFade()
61                .into(dialog_urlImage);
62
63        builder.setView(view);
64        builder.setNeutralButton("ok", new DialogInterface.OnClickListener() {
65            @Override
66            public void onClick(DialogInterface dialog, int which) {
67
68            }
69        });
70        builder.show();
71    }

xml
1
2/* dialog_layout.xml */
3
4<?xml version="1.0" encoding="utf-8"?>
5<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
6    android:orientation="vertical" android:layout_width="match_parent"
7    android:layout_height="match_parent"
8    android:id="@+id/root_layout"
9    >
10
11    <ImageView
12        android:layout_width="fill_parent"
13        android:layout_height="fill_parent"
14        android:id="@+id/showing_image"/>
15
16</LinearLayout>