Android databaseOpenHelper returning null

i am trying to access my database of projects.but no able make connection with database here i am posting three different code by this control is traversing

MainActivity.java

 int id = item.getItemId();
    projectDetails project_list=new projectDetails();

    if (id == R.id.nav_javaprjct)
    {
        project_list.getProjects("java");
        Intent intent=new Intent(this,projectDetails.class);

    } else if (id == R.id.nav_netprjct) {

DatabaseAccess.java

public class DatabaseAccess {
private SQLiteOpenHelper openHelper;
public SQLiteDatabase database;
private static DatabaseAccess instance;
public static String TAG="Database Access class";

public DatabaseAccess(Context context)
{

    this.openHelper = new DatabaseOpenHelper(context);

}

public static DatabaseAccess getInstance(Context context) {


    if (instance == null)
    {
        instance = new DatabaseAccess(context);
    }
    return instance;
}

public void open() {
    this.database = openHelper.getWritableDatabase();
}

public void close() {
    if (database != null) {
        this.database.close();
    }
}

public List<String> getProjectList(String techname)
{
    List<String> list=new ArrayList<>();

    if(techname.equals("java"))
    {
        Cursor cursor = database.rawQuery("SELECT JProject_Name FROM javaproject", null);
        cursor.moveToFirst();

        while (!cursor.isAfterLast()) {
            list.add(cursor.getString(0));
            cursor.moveToNext();
        }
        cursor.close();
    }
    else
    {

    }
    return list;

}

}

DatabaseOpenHelper.java

public class DatabaseOpenHelper extends SQLiteAssetHelper {
private static final String DATABASE_NAME = "totalbinary_db.db";
private static final int DATABASE_VERSION = 1;

public DatabaseOpenHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);

}

}

stack trace

01-15 04:45:36.514 15450-15450/totalbinary.shailesh.myapplication       I/Choreographer: Skipped 72 frames!  The application may be doing too much work on its main thread.
***01-15 04:45:39.064 15450-15450/totalbinary.shailesh.myapplication D/Project Detail class: DB Error: java.lang.NullPointerException***