Notice: Undefined index: page

Notice: Undefined index: page in wpsp-terms-of-use.php on line 142

/* Display Attention Message to user until they do the settings for the plugin */
function tou_admin_notice(){
$page_name = $_GET['page'];
	if(get_option('wpsp_tou_msg') == '0') {
		if($page_name == 'wpsp-terms-of-use') {
		} else {
    echo '<div class="updated">
       <p><strong>Attention:</strong> Please visit the WPSP Terms of Use <a href="'.get_admin_url().'options-general.php?page=wpsp-terms-of-use">Settings</a> page to setup the plugin.</p>
    </div>';
    	}
    }
}

Following line (in code as mentioned above) seems to be the reason for it (is line 142):
$page_name = $_GET['page'];

Maybe someone knows how to solve this, it is a very nice plugin but seems not to be updated to latest WP version.

Thanks in advance for time and effort.