Some points you may want to consider:
1. .htaccess and/or the snippet which @redsand provides are both only a valid solution if and only if, the error would be caused by the file being called directly – which most often would happen as a potential hack attempt.
Without a backtrace of the error there is however no way to determine if that’s the cause of the notice showing up.
To get a backtrace, you can use this code – do follow the inline instructions carefully!
2. All the code at the top of that file will be executed in the global namespace at file load. Better to run it at the plugins_loaded hook, i.e. you could add nearly all of that code to the start of the aioseop_init_class function without any change in functionality (constants are always global even when defined from within a function).
3. You may also want to wrap the add_action() call to plugins_loaded within a check for plugin installing as that sometimes gives issues too.
Like so:
if ( ! defined( 'WP_INSTALLING' ) || WP_INSTALLING === false ) {
add_action( 'plugins_loaded', 'aioseop_init_class' );
}
