E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.hp.marklist, PID: 28522
              java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.trim()' on a null object reference
                  at com.example.hp.marklist.admin$1LoginAsync.onPostExecute(admin.java:143)
                  at com.example.hp.marklist.admin$1LoginAsync.onPostExecute(admin.java:81)
                  at android.os.AsyncTask.finish(AsyncTask.java:651)
                  at android.os.AsyncTask.-wrap1(AsyncTask.java)
                  at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:5417)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

package com.example.hp.marklist;

import android.app.Dialog;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;


import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;


import java.io.UnsupportedEncodingException;
import java.util.ArrayList;
import java.util.List;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.util.ArrayList;
import java.util.List;

public class admin extends AppCompatActivity {


public static final String USER_NAME="USERNAME";
public EditText username;
public EditText password;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_admin);

    Button l=(Button)findViewById(R.id.log);
    l.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
           // Toast t=new Toast()
            EditText username=(EditText)findViewById(R.id.username);
            EditText password=(EditText)findViewById(R.id.password);

            String use;
            String pas;
            use = username.getText().toString();
            pas = password.getText().toString();
            login(use,pas);

            Toast.makeText(getApplicationContext(), "Clicked!!!", Toast.LENGTH_LONG).show();
        }
    });


}
public void invokeLogin(View view){
   /* use = username.getText().toString();
    pas = password.getText().toString();

    login(use,pas);*/

}

private void login(final String username, String password) {

    class LoginAsync extends AsyncTask<String, Void, String> {

        private Dialog loadingDialog;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            loadingDialog = ProgressDialog.show(admin.this, "Please wait", "Loading...");
        }

        @Override
        protected String doInBackground(String... params) {
            String uname = params[0];
            String pass = params[1];
            //loadingDialog.dismiss();
            //


            //
            InputStream is = null;
            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
            nameValuePairs.add(new BasicNameValuePair("username", uname));
            nameValuePairs.add(new BasicNameValuePair("password", pass));
            //String result="";
            String result = null;

            try{
                HttpClient httpClient = new DefaultHttpClient();
                HttpPost httpPost = new HttpPost(
                        "http://192.168.1.5/logina.php");
                httpPost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

                HttpResponse response = httpClient.execute(httpPost);

                 HttpEntity entity = response.getEntity();

                is = entity.getContent();

                BufferedReader reader = new BufferedReader(new InputStreamReader(is, "UTF-8"), 8);
                StringBuilder sb = new StringBuilder();

                String line = null;
                while ((line = reader.readLine()) != null)
                {
                    sb.append(line + "n");
                }

                 result =sb.toString();
            } catch (ClientProtocolException e) {
                e.printStackTrace();
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            //Toast.makeText(getApplicationContext(), "got it!!!"+result, Toast.LENGTH_LONG).show();
            return result;
        }

        @Override
        protected void onPostExecute(String result){
            Toast.makeText(getApplicationContext(), "got it!!!", Toast.LENGTH_LONG).show();
            String s = result.trim();
            //String s=result;
            Toast.makeText(getApplicationContext(),result, Toast.LENGTH_LONG).show();
            loadingDialog.dismiss();
            Toast.makeText(getApplicationContext(), s, Toast.LENGTH_LONG).show();
            if(s.equalsIgnoreCase("success")){
                Toast.makeText(getApplicationContext(), "got it!!!", Toast.LENGTH_LONG).show();
                //Intent intent = new Intent(LogA.this, user.class);
                //intent.putExtra(USER_NAME, username);
                finish();
                //break;
                //startActivity(intent);
            }else {
                Toast.makeText(getApplicationContext(), s, Toast.LENGTH_LONG).show();
                Toast.makeText(getApplicationContext(), "Invalid User Name or Password"+s, Toast.LENGTH_LONG).show();
            }
        }
    }

    LoginAsync la = new LoginAsync();
    la.execute(username, password);

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    //getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    //if (id == R.id.action_settings) {
    return true;
    //}

    //return super.onOptionsItemSelected(item);
}
}

<?php
require_once 'Config.php';
$con = mysqli_connect(DB_HOST,DB_USER,DB_PASSWORD,DB_DATABASE);
$username = $_POST["username"];
$password = $_POST["password"];

$sql = "select * from admindb where username='$username' and   password='$password'";
$res = mysqli_query($con,$sql);
$check = mysqli_fetch_array($res);
if(isset($check))
echo 'success';
else
echo 'failure';
mysqli_close($con);
?>