Unique Case of "Call to a member function set_type() on null" - debugging

Question

Hello guys i have gone through all 25 questions relating to the title of my question and i am made to believe my scenario is totally different.
Here is the error
Fatal error: Uncaught Error: Call to a member function set_type() on null in /path_to_file/login.php:55 Stack trace: #0 /path_to_call_page/login.php(24): login->authenticate() #1 {main} thrown in /path_to_file/login.php on line 55
Here is my code
<?php
class login
{
public $username;
public $password;
public $err;
public $table_name;
public $session_data;
public $md5 = true;
public $username_column = "username";
public $password_column = "password";
public $builder;
function _construct($username,$password)
{
$this->username = $username;
$this->password = $password;
$this->builder = new queryBuilder();
}
function set_table($tablename)
{
$this->table_name = $tablename;
}
/*
* Tells the login class where to find the username and password in database table
*/
function set_columns($username_col,$password_col)
{
$this->username_column = $username_col;
$this->password_column = $password_col;
}
function md5_on()
{
$this->md5 = true;
}
function md5_off()
{
$this->md5 = false;
}
function authenticate()
{
$db = new mySQLConnection();
$db->select();
// if md5 is turned on
if($this->md5)
$this->password = md5($this->password);
$this->builder->set_type("SELECT");
$this->builder->set_table_name($this->table_name);
$this->builder->set_where("WHERE $this->username_column = '$this->username' and $this->password_column = '$this->password'");
$query = $this->builder->build_query();
if($db->execute_query($query))
$data = $db->fetch($db->result);
else
{
die($db->error);
}
if($db->rows_affected() == 1)
{
$this->session_data = $data[0];
return true;
}
else
{
$this->err = "Invalid username or password. Please try again";
return false;
}
}
function get_error()
{
return $this->err;
}
}
?>
The error occurs everywhere i have
$this->builder
And i have defined it in the _construct method.
This is the queryBuilder class
class queryBuilder
{
var $data;
var $field;
var $tableName;
var $databaseName;
var $where;
var $order;
var $group;
var $limit;
var $queryString;
var $error;
private function put_quotes1($field)
{
$field = trim($field);
$field = "`".$field."`";
return $field;
}
private function put_quotes2($field)
{
$field = trim($field);
$field = "'".$field."'";
return $field;
}
function set_type($type)
{
$this->type = $type;
}
function set_data($data)
{
$this->data = $data;
}
function set_field($field = null)
{
$this->field = $field;
}
function set_where($where)
{
$this->where = $where;
}
function set_limit($limit)
{
$this->limit = $limit;
}
function set_order($order)
{
$this->order = $order;
}
function set_table_name($name)
{
$this->tableName = $name;
}
function prepare_data($data)
{
if(is_array($data))
{
foreach($data as $k => $v)
{
$this->field[] = $k; //setting the column names
$this->data[] = $v; // setting the values
}
}
}
function build_query()
{
switch($this->type)
{
case 'SHOW':
$database_name = $this->put_quotes1($this->databaseName);
$this->queryString = "SHOW ";
if(!isset($this->field) || is_null($this->field))
$this->queryString .= "DATABASES LIKE 'thirdeye%'; ";
else{
$noFields = count($this->field); //no of fields in table
for($i = 0; $i < $noFields; $i++)
{
if($i == ($noFields- 1)) // if on the last field
$this->queryString .= $this->put_quotes1($this->field[$i]).' ';
else
$this->queryString .= $this->put_quotes1($this->field[$i]).',';
}
}
break;
case 'INSERT':
$table_name = $this->put_quotes1($this->tableName);
$this->queryString = "INSERT INTO ".$table_name." (";
$noFields = count($this->field);
$noData = count($this->data);
if($noFields > 0 && $noData > 0)
{
for($i = 0; $i < $noFields; $i++)
{
if($i == ($noFields- 1))
$this->queryString .= $this->put_quotes1($this->field[$i]).')';
else
$this->queryString .= $this->put_quotes1($this->field[$i]).',';
}
$this->queryString.= " VALUES (";
for($i = 0; $i < $noData; $i++)
{
if($i == ($noData -1))
$this->queryString .= $this->put_quotes2($this->data[$i]).');';
else
$this->queryString .= $this->put_quotes2($this->data[$i]).',';
}
}
else
{
$this->error = "No column name or data was supplied";
}
break;
case 'SELECT':
$table_name = $this->put_quotes1($this->tableName);
$this->queryString = "SELECT ";
if(!isset($this->field) || is_null($this->field))
$this->queryString .= "* ";
else{
$noFields = count($this->field); //no of fields in table
for($i = 0; $i < $noFields; $i++)
{
if($i == ($noFields- 1)) // if on the last field
$this->queryString .= $this->put_quotes1($this->field[$i]).' ';
else
$this->queryString .= $this->put_quotes1($this->field[$i]).',';
}
}
$this->queryString .= "FROM ".$table_name;
if(isset($this->where))
$this->queryString .= " ".$this->where;
if(isset($this->order))
$this->queryString .= " ".$this->order;
if(isset($this->limit))
$this->queryString .= " ".$this->limit;
else
$this->queryString .= ";";
break;
case 'UPDATE':
$table_name = $this->put_quotes1($this->tableName);
$this->queryString = "UPDATE ". $table_name. " SET ";
$noFields = count($this->field); //no of fields in table
if(is_array($this->field) && is_array($this->data) && isset($this->where))
{
for($i = 0; $i < $noFields; $i++)
{
if($i == ($noFields -1))
$this->queryString .= $this->put_quotes1($this->field[$i])." = ". $this->put_quotes2($this->data[$i]).' ';
else
$this->queryString .= $this->put_quotes1($this->field[$i])." = ". $this->put_quotes2($this->data[$i]).',';
}
$this->queryString .= " ".$this->where.";";
}
else
{
$this->error = "Cannot build query. One of the following was not set";
}
break;
case 'DELETE':
$table_name = $this->put_quotes1($this->tableName);
$this->queryString = "DELETE FROM ".$table_name;
if(isset($this->where))
{
$this->queryString .= " ".$this->where.";";
}
else
{
$this->error = "Connot build. No condition was set";
}
break;
}
return $this->queryString;
}
}
Any pointers would help. Remember i have been through previous questions so a suggested edit or code answer would be great.