error at Attempt to invoke virtual method

Question

Hello. I am getting the error "Attempt to invoke virtual method DatabaseHandler.open() on a null object reference. I cannot seem to solve this problem. Any help would be greatly appreciated!!!!

Here is my code inside my DatabaseHandler.java

 

package com.ebookfrenzy.finalproject1;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

/**
 * Created by Matt_1 on 2016-04-04.
 */
public class DatabaseHandler {
    public static final String NAME="name";
    public static final String ROLL="roll";
    public static final String COURSE="course";
    public static final String TABLE_NAME="studenttable";
    public static final String DATABASE_NAME="studentdb";
    public static final int DATABASE_VERSION=1;
    //Table query
 public static final String TABLE_CREATE="create table studenttable(name text not null, roll text not null, course text not null);";
    DataBaseHelper dbhelper;
    Context context;
    SQLiteDatabase db;
    public DatabaseHandler(Context ctx)
    {
        this.context=ctx;
        dbhelper=new DataBaseHelper(context);
    }
    private static class DataBaseHelper extends SQLiteOpenHelper
    {
        
 public DataBaseHelper(Context ctx)
        {
            super(ctx,DATABASE_NAME,null,DATABASE_VERSION);
        }
        @Override
 
 public void onCreate(SQLiteDatabase db)
        {
            db.execSQL(TABLE_CREATE);
 }
        @Override

 public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            db.execSQL("DROP TABLE IF EXIST studenttable");
            onCreate(db);
        }
    }
    public DatabaseHandler open()
    {
        
 db=dbhelper.getWritableDatabase();
        return this;
    }
    public void close()
    {
 dbhelper.close();
    }
    //Insert record in the database
 public long InsertData(String name, String roll,String course)
    {
        
 ContentValues content=new ContentValues();
        content.put(NAME, name);
        content.put(ROLL, roll);
        content.put(COURSE, course);//Adds a value to the set.
 return db.insertOrThrow(TABLE_NAME,null, content);
    }
    //Display record from the database
 public Cursor DisplayData()
    {
        //Select query
 return db.rawQuery("SELECT * FROM studenttable", null);
        //return db.query(TABLE_NAME, new String[]{NAME, ROLL,COURSE}, null, null, null, null, null);
 }
}

Again I would appreciate any help to me!! EDIT: Also, here is my MainActivity.java

package com.ebookfrenzy.finalproject1;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.EditText;
import android.widget.GridView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    private EditText nameEditText;
    private EditText rollEditText;
    private EditText courseEditText;
    private Button insertButton;
    private Button displayButton;
    DatabaseHandler handler;

    @Override
 protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        nameEditText=(EditText) findViewById(R.id.NameEditText);
        rollEditText=(EditText) findViewById(R.id.RollEditText);
        courseEditText=(EditText) findViewById(R.id.CourseEditText);
        insertButton=(Button) findViewById(R.id.InsertButton);
        displayButton=(Button) findViewById(R.id.DisplayButton);
        insertButton.setOnClickListener(new View.OnClickListener() {
            @Override
 public void onClick(View v) {
                String name = nameEditText.getText().toString();
                String roll = rollEditText.getText().toString();
                String course = courseEditText.getText().toString();
                //getting the context object
 handler.open();
                long id = handler.InsertData(name, roll, course);
                Toast.makeText(getBaseContext(), "Your data in inserted", Toast.LENGTH_LONG).show();
                nameEditText.setText("");
                rollEditText.setText("");
                courseEditText.setText("");
                handler.close();
            }
        });
        displayButton.setOnClickListener(new View.OnClickListener() {
            @Override
 public void onClick(View v) {
                Intent i = new Intent(MainActivity.this, GridView.class);
                startActivity(i);//start gridview activity to show the records.
 }
        });

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
 public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @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);
    }
}