java.lang.NullPointerException at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)

I have created an app that stores certain data in database(SQLite) but i am facing a problem that while storing the data i get NullPointerException, (Logcat attached). the problem i m facing is at SQLiteDatabse db=this.getWritableDatabase();

I have searched for almost all solution but none helped,If anyone could help it would be a great help.

Thanks

package com.example.neuronsdk;

import android.content.ContentValues;
import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;


public class DatabaseHandler extends SQLiteOpenHelper {

    private static final int DATABASE_VERSION = 2;
    private static final String DATABASE_NAME = "NeuronDatabase";

    private static final String TABLE_EVENTS = "Events";
    private static final String TAG = "DatabaseHandler.class";
    static Context context;
    private static final String KEY_ID = "id";
    private static final String KEY_EVENTS = "eventsText";
    private static final String KEY_START_TIME="start_time";
    private static final String KEY_END_TIME="end_time";

    String url="http://ift.tt/1LlAtcG";

    SQLiteDatabase dbHelper=null;
    DatabaseHandler dbh;





    private static final String NEURON_DATABASE ="CREATE TABLE " + TABLE_EVENTS + "("   +
                                                                KEY_EVENTS + " TEXT NOT NULL, " + KEY_START_TIME + " INTEGER," +  
                                                                KEY_END_TIME + " INTEGER );";                   
    public DatabaseHandler(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        db.execSQL(NEURON_DATABASE);
        Log.d(TAG, "Database Created"); 
        }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
         db.execSQL("DROP TABLE IF EXISTS " + TABLE_EVENTS);
         this.onCreate(db);
    }

     public void open() throws SQLException {
         close();
        Context context = null;
        //this.context=context;
        dbh=new DatabaseHandler(context);
        dbHelper = dbh.getWritableDatabase();
        }

    public void addEvent(String eventText){
        ContentValues values = new ContentValues();
        Log.d("Database", "Adding value in database"+ eventText);
        open();
        if(eventText != null)
        {
        values.put(KEY_EVENTS, eventText);
        dbHelper.insert(TABLE_EVENTS, null, values);
        close();
        }
    }

    public void addStartTime(Long startTime){
        ContentValues values = new ContentValues();
        Log.d("Database", "Adding start time in database"+ startTime);
        close();
        //dbHelper=this.getWritableDatabase();
        values.put(KEY_START_TIME, startTime);
        dbHelper.insert(TABLE_EVENTS, null, values);    
        close();
    }

    public void addEndTime(Long endTime){
        ContentValues values = new ContentValues();
        Log.d("Database", "Adding End time in database"+ endTime);
        close();
    //  dbHelper=getWritableDatabase();
        values.put(KEY_END_TIME, endTime);
        dbHelper.insert(TABLE_EVENTS, null, values);
        close();
        }
}

Error Report of Logcat is :

07-16 07:08:32.435: E/AndroidRuntime(1869): FATAL EXCEPTION: main
07-16 07:08:32.435: E/AndroidRuntime(1869): Process: com.example.test_application, PID: 1869
07-16 07:08:32.435: E/AndroidRuntime(1869): java.lang.NullPointerException
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.neuronsdk.DatabaseHandler.open(DatabaseHandler.java:59)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.neuronsdk.DatabaseHandler.addEvent(DatabaseHandler.java:72)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.neuronsdk.NeuronActivity.onClick(NeuronActivity.java:28)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.test_application.MainActivity$1.onClick(MainActivity.java:47)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.view.View.performClick(View.java:4438)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.view.View$PerformClick.run(View.java:18422)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.os.Handler.handleCallback(Handler.java:733)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.os.Handler.dispatchMessage(Handler.java:95)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.os.Looper.loop(Looper.java:136)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.app.ActivityThread.main(ActivityThread.java:5017)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at java.lang.reflect.Method.invokeNative(Native Method)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at java.lang.reflect.Method.invoke(Method.java:515)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at dalvik.system.NativeStart.main(Native Method)