SQLite Android null pointer exception

I keep receiving the following error when running my app and attempting to add a new User to my system.

java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference
        at com.dissertation.anna.rpiinventory.UsersDAO.createUser(UsersDAO.java:60)
        at com.dissertation.anna.rpiinventory.Register.onClick(Register.java:68)

The UsersDAO class:

public class UsersDAO {

//DB fields
private SQLiteDatabase aDatabase;
private DatabaseHelper aDbHelper;
Context aContext;
private String[] uAllColumns = {DatabaseHelper.COLUMN_USER_ID, DatabaseHelper.COLUMN_UNI_ID,
        DatabaseHelper.COLUMN_USER_NAME, DatabaseHelper.COLUMN_USER_EMAIL, DatabaseHelper.COLUMN_USER_COURSE,
        DatabaseHelper.COLUMN_USER_TYPE, DatabaseHelper.COLUMN_ADMIN_PASSWORD};


public UsersDAO(Context context){

    this.aContext = context;
    aDbHelper = new DatabaseHelper(context);


    //open DB
    try{
        open();
    } catch (SQLException e){
        Log.e(TAG, "SQLException on opening Database " + e.getMessage());
        e.printStackTrace();
    }
}

public void open() throws SQLException{
    aDatabase = aDbHelper.getWritableDatabase();
}

public void close(){
    aDbHelper.close();
}

public Users createUser(String uniID, String name, String email, String course, String type, String password) {
    ContentValues values = new ContentValues();
    values.put(DatabaseHelper.COLUMN_UNI_ID, uniID);
    values.put(DatabaseHelper.COLUMN_USER_NAME, name);
    values.put(DatabaseHelper.COLUMN_USER_EMAIL, email);
    values.put(DatabaseHelper.COLUMN_USER_COURSE, course);
    values.put(DatabaseHelper.COLUMN_USER_TYPE, type);
    values.put(DatabaseHelper.COLUMN_ADMIN_PASSWORD, password);

    long insertID = aDatabase.insert(DatabaseHelper.TABLE_USERS, null, values);
    Cursor cursor = aDatabase.query(DatabaseHelper.TABLE_USERS, uAllColumns, DatabaseHelper.COLUMN_USER_ID
            + " = " + insertID, null, null, null, null);
    cursor.moveToFirst();
    Users newUser = cursorToUser(cursor);
    cursor.close();
    return newUser;
}

The onClick method in the Register class

  public void onClick(View v){
    switch (v.getId()){
        case R.id.register_OK_button:
            Editable u_adminName= adminName.getText();
            Editable u_adminEmail = adminEmail.getText();
            Editable u_adminNumber = adminNumber.getText();
            Editable u_adminPassword = adminPassword.getText();

            String u_userType = "Admin";
            String u_course = "N/A";

            String idNumber = u_adminNumber.toString();
            String name = u_adminName.toString();
            String email = u_adminEmail.toString();
            String password = u_adminPassword.toString();

            if (!TextUtils.isEmpty(u_adminName) && !TextUtils.isEmpty(u_adminEmail)
                    && !TextUtils.isEmpty(u_adminNumber) && !TextUtils.isEmpty(u_adminPassword)
                    && !TextUtils.isEmpty(u_userType) && !TextUtils.isEmpty(u_course)){

                //Add Admin to Database
               Users createdAdmin = u_adminDAO.createUser(idNumber, name,
                      email, u_course,u_userType, password);

                Log.d(TAG, "Created admin : " + createdAdmin.getFullName());
                Intent intent = new Intent();
                intent.putExtra(ListAdminsActivity.EXTRA_ADDED_ADMIN, createdAdmin);
                setResult(RESULT_OK, intent);
                Toast.makeText(this, R.string.admin_added_successfully, Toast.LENGTH_LONG).show();
                finish();
            }//if
            else{
                Toast.makeText(this, "One or more fields are empty.", Toast.LENGTH_LONG).show();
            }//else
            break;

        default:
            break;
    }//switch
}//onClick

Database Handler:

private static final String DATABASE_NAME = "rpi_inventory_new.db";
private static final int DATABASE_VERSION = 1;

//SQL STATEMENT OF USERS TABLE CREATION
private static final String SQL_CREATE_TABLE_USERS = "CREATE TABLE " + TABLE_USERS + "("
        + COLUMN_USER_ID + " INTEGER AUTOINCREMENT, "
        + COLUMN_UNI_ID + " TEXT PRIMARY KEY, "
        + COLUMN_USER_NAME + " TEXT NOT NULL, "
        + COLUMN_USER_EMAIL + " TEXT NOT NULL, "
        + COLUMN_USER_COURSE + " TEXT NULL, "
        + COLUMN_USER_TYPE + " TEXT NOT NULL, "
        + COLUMN_ADMIN_PASSWORD + " PASSWORD NULL"
        + ");";

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

@Override
public void onCreate(SQLiteDatabase database){
    database.execSQL(SQL_CREATE_TABLE_USERS);
    database.execSQL(SQL_CREATE_TABLE_BORROWING);
    database.execSQL(SQL_CREATE_TABLE_KITS);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
    Log.w(TAG,
            "Upgrading the database from version " + oldVersion + " to " + newVersion);

    db.execSQL("DROP TABLE IF EXISTS " + TABLE_USERS);
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_BORROWING);
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_KITS);

    onCreate(db);
}

public DatabaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version){
    super(context, DATABASE_NAME, factory, DATABASE_VERSION);
}

}

I have checked various other answers quite thoroughly and none of the answers have worked on my database. I have no idea why it is occurring or what I am doing wrong, I have created another similar database and it works just fine.

The two lines causing the errors are:

    long insertID = aDatabase.insert(DatabaseHelper.TABLE_USERS, null, values);

And:

    Users createdAdmin = u_adminDAO.createUser(idNumber, name,
                      email, u_course,u_userType, password);