public function getOAuthToken()
    {
        // If any of the required fields are empty, return null.
        $hasAnEmptyField = empty($this->northstar_id) || empty($this->access_token) ||
            empty($this->access_token_expiration) || empty($this->refresh_token) || empty($this->role);

        if ($hasAnEmptyField) {
            return null;
        }

        return new AccessToken([
            'resource_owner_id' => $this->getNorthstarIdentifier(),
            'access_token' => $this->access_token,
            'refresh_token' => $this->refresh_token,
            'expires' => $this->access_token_expiration,
            'role' => $this->role,
        ]);
    }