Unexpected error with class-wp-hook.php on WordPress

Question

Somewhere in the theme, the WP_Maintenance_Mode class method is hooked to the filter or action.
But when the hook is executed, this class doesn’t exists or is not yet included.

I suspect that in the theme (file functions.php) this class is hooked to some event, but it is defined in an inactive plugin (I’m guessing it is “WP Maintenance Mode”.)

Look for add_action and add_filter where this class is used as parameter.

add_action( 'some_action_name',  array('WP_Maintenance_Mode', 'method_name') );
add_action( 'some_action_name',  'WP_Maintenance_Mode::method_name' );
add_filter( 'some_filter_name',  array('WP_Maintenance_Mode', 'method_name') );

Try to comment out these lines or install the mentioned plugin and check if the warning disappears.