Trying to access array offset on value of type null in The php

public function isStudent($username_to_check) {
    $usernameComma = "," . $username_to_check . ",";

    if(strstr($this->user2['student_array'], $usernameComma) || $username_to_check == $this->user2['username']) {
        return true;
    }
    elseif(strstr($this->code['student_array'], $usernameComma)){
        return true;
    }
    else {
        return true; // it could be done in another method... which is tiresome
    }
}

}

C:\xampp\htdocs\classroom\includes\classes\User.php on line 106 This is The error from the File