Error message when saving a page or menu

When saving the Appearance > Menu page I get this

Notice: Trying to get property of non-object in .../wp-content/plugins/slide-in/lib/class_wdsi_slide_in.php on line 569

Notice: Undefined index: post_ID in .../wp-content/plugins/wpmu-dev-facebook/lib/class_wdfb_admin_pages.php on line 810

Notice: Undefined index: post_ID in .../wp-content/plugins/wpmu-dev-facebook/lib/class_wdfb_admin_pages.php on line 810

What does that mean and how do we fix it?

Thanks