Null pointer exception while inserting data into sqlite using android

I am getting null pointer exception while inserting values into sqlite database. The dh in DatabaseHelper dh = new DatabaseHelper(this) is coming out to be null when I try to check its value by putting a breakpoint. Below is the code of my DatabaseHelper class and the class from where I am trying to insert the values.

DatabaseHelper Class:

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

public class DatabaseHelper extends SQLiteOpenHelper {

private static final String DATABASE_NAME="game.db";
public static final String USER_ID="userId";
public static final String STATUS="status";
public static final String MINES="mines";
public static final String OPENTILE="openTile";

public DatabaseHelper(Context context)
{
    super(context,DATABASE_NAME,null,1);
}
@Override
public void onCreate(SQLiteDatabase arg0) {
    // TODO Auto-generated method stub
    //CREATE TABLE minesweeper(_id INTEGER PRIMARY KEY AUTOINCREMENT,userId TEXT
    arg0.execSQL("CREATE TABLE minesweeper(_id INTEGER PRIMARY KEY AUTOINCREMENT,userId TEXT,status TEXT, mines TEXT, openTile TEXT);");

}

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

}

Main Class:

package com.app.game;
import android.app.Activity;
import android.content.ContentValues;
import android.content.Intent;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.util.Log;
import android.view.KeyEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.TextView;

public class AppMain extends Activity implements OnClickListener {
   String intentString;

   DatabaseHelper dha;

   /** Called when the activity is first created. */
   @Override
   public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.main);

      dha = new DatabaseHelper(this);

      //insertMinesweeper(intentString);
   }

   void insertMinesweeper(String userId, String status, String mines, String openTile) {
       Log.d("","Inside Database function");

       Log.d("","Done with database helper");
       Log.d("", status);
       Log.d("", mines);
       Log.d("", openTile);
       try
       {
           //DatabaseHelper dh = new DatabaseHelper(this);
       SQLiteDatabase db = dha.getWritableDatabase();
       Log.d("","Done with database Sqlite");
       ContentValues cv = new ContentValues();
       Log.d("","Database connected");
       cv.put(DatabaseHelper.USER_ID, intentString);
       cv.put(DatabaseHelper.STATUS, status);
       cv.put(DatabaseHelper.MINES, mines);
       cv.put(DatabaseHelper.OPENTILE, openTile);
       db.insert("minesweeper", DatabaseHelper.USER_ID, cv);
       db.close();
       Log.d("","Values inserted"); }

       catch(SQLException e)
       {
           e.printStackTrace();
       }

       catch(NullPointerException e) {
            e.printStackTrace();
        }
    // TODO Auto-generated method stub

}



   }

}

insertMinesweeper() is being called from another class.

Please help. Thanks in advance.