Adding CSS to custom post type admin page causes error

You need to check for the presence of 'post_type' as an index of $_GET before using it:

if ((isset($_GET['post_type']) && $_GET['post_type'] == 'slide') || (isset($post_type) && $post_type == 'slide')) :

Also, you should be using the wp_enqueue_style function instead of echoing your stylesheet at 'admin_head':

wp_enqueue_style( 'meteor-slides-admin', plugins_url('/css/meteor-slides-admin.css', __FILE__), array(), '1.0' );

More information on wp_enqueue_style here.

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