php - Call to a member function

Please, how do I resolve the following
error?

Fatal
error: Call to a member function fetch_assoc() on a non-object

in......

database.php

protected
            function get_results_from_query()
            {
$this->open_connection();
$result =
            $this->conn->query($this->query);
while ($this->rows[] =
            $result->fetch_assoc());
$result->close();
$this->close_connection();
array_pop($this->rows);
}
            

usuario.php

public
            function getUsers(){
//$this->query variable define in class abstract
            database 
$this->query = " SELECT * FROM Usuario LIMIT 0,
            30";
$this->get_results_from_query();
}

Everything
is working perfect, but calling function getUsers results in
"fatal error".