Gsonを使ったjsonのパースがうまくいかない

以下のコードを使って以下のjsonから値を取得しようとしています．

{
"block_size":50,
"mosaic_size_h":45,
"mosaic_size_w":80,
"0":"0",
"1":"3",
"2":"3",
"3":"3",
"4":"3",
...
}

import android.util.Log;
import com.google.gson.Gson;
import com.google.gson.JsonObject;
import java.util.Arrays;

public class CreateMosaic {
    public void CreateMosaicArt(String pMosaic){
        int size=0;
        int w=0;
        int h=0;
        Log.d("json",pMosaic);

        JsonObject jsonObject=(JsonObject) new Gson().fromJson(pMosaic,JsonObject.class);

        size=jsonObject.getAsJsonObject().get("block_size").getAsInt();
        w=jsonObject.getAsJsonObject().get("mosaic_size_w").getAsInt();
        h=jsonObject.getAsJsonObject().get("mosaic_size_h").getAsInt();
        int imgNum[]=new int[w*h];
        for (int i=2;i<jsonObject.size();i++){
            imgNum[i]=jsonObject.getAsJsonObject().get(String.valueOf(i)).getAsInt();
        }
        Log.d("size",String.valueOf(size));
        Log.d("w",String.valueOf(w));
        Log.d("h",String.valueOf(h));
        Log.d("imgs", Arrays.toString(imgNum));

    }

}

しかし以下のようなエラーが出ます

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.tadatouta.downloadtest, PID: 14807
    java.lang.NullPointerException: Attempt to invoke virtual method 'int com.google.gson.JsonElement.getAsInt()' on a null object reference
        at com.example.tadatouta.downloadtest.CreateMosaic.CreateMosaicArt(CreateMosaic.java:30)
        at com.example.tadatouta.downloadtest.MainActivity$1.CallBack(MainActivity.java:25)
        at com.example.tadatouta.downloadtest.DownloadTask.onPostExecute(DownloadTask.java:93)
        at com.example.tadatouta.downloadtest.DownloadTask.onPostExecute(DownloadTask.java:13)
        at android.os.AsyncTask.finish(AsyncTask.java:695)
        at android.os.AsyncTask.-wrap1(Unknown Source:0)
        at android.os.AsyncTask$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:6494)
        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:807)
Application terminated.

どうすれば正しく動きますか？