How to recover null pointer exception error in android studio?

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Java

Android

package com.example.frogs.json;

import android.app.ProgressDialog;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import com.android.volley.Request;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.VolleyLog;
import com.android.volley.toolbox.JsonArrayRequest;
import com.android.volley.toolbox.JsonObjectRequest;

import org.json.JSONArray;
import org.json.JSONObject;

public class MainActivity extends AppCompatActivity {
Button btnJson;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);

// Tag used to cancel the request
String tag_json_arry = "json_array_req";

String url = "http://api.androidhive.info/volley/person_array.json";

final ProgressDialog pDialog = new ProgressDialog(this);
pDialog.setMessage("Loading...");
pDialog.show();

JsonArrayRequest req = new JsonArrayRequest(url,
new Response.Listener<jsonarray>() {
@Override
public void onResponse(JSONArray response) {
Log.d("", response.toString());
pDialog.hide();
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
VolleyLog.d("", "Error: " + error.getMessage());
pDialog.hide();
}
});

// Adding request to request queue
AppController.getInstance().addToRequestQueue(req, tag_json_arry);

String string ="{\"stats\" :true , \"Error\" :\"\",\"NO_OF_USER\" :50}";
btnJson=(Button)findViewById(R.id.btnJson);
btnJson.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// Tag used to cancel the request
String tag_json_obj = "json_obj_req";

String url = "http://api.androidhive.info/volley/person_object.json";

final ProgressDialog pDialog = new ProgressDialog(MainActivity.this);
pDialog.setMessage("Loading...");
pDialog.show();

JsonObjectRequest jsonObjReq = new JsonObjectRequest(Request.Method.GET,
url, null,
new Response.Listener<jsonobject>() {

@Override
public void onResponse(JSONObject response) {
Log.d("", response.toString());
pDialog.hide();
}
}, new Response.ErrorListener() {

@Override
public void onErrorResponse(VolleyError error) {
VolleyLog.d("", "Error: " + error.getMessage());
// hide the progress dialog
pDialog.hide();
}
});

// Adding request to request queue
AppController.getInstance().addToRequestQueue(jsonObjReq, tag_json_obj);
}
});




}

}

What I have tried:

i try to connect the server but i can't be kindly asap send me the solution?