Slideshows: don't attempt to use the block when not available (#38744)

* Slideshows: don't attempt to use the block when not available

Fixes #38742

* Load the slideshow block manually if needed.

projects/plugins/jetpack/changelog/fix-slideshow-block-non-existing-shortcode

@@ -0,0 +1,4 @@
+Significance: patch
+Type: compat
+
+AMP: avoid errors when using Jetpack's classic slideshows on a site where Jetpack blocks are disabled.

projects/plugins/jetpack/modules/shortcodes/slideshow.php

@@ -183,7 +183,10 @@ public function shortcode_callback( $attr ) {
 			);
 		}
 
-		if ( class_exists( 'Jetpack_AMP_Support' ) && Jetpack_AMP_Support::is_amp_request() ) {
+		if (
+			class_exists( 'Jetpack_AMP_Support' )
+			&& Jetpack_AMP_Support::is_amp_request()
+		) {
 			// Load the styles and use the rendering method from the Slideshow block.
 			Jetpack_Gutenberg::load_styles_as_required( 'slideshow' );
 
@@ -195,6 +198,14 @@ public function shortcode_callback( $attr ) {
 				$amp_args['autoplay'] = true;
 			}
 
+			/*
+			 * Blocks can be disabled in Jetpack Settings.
+			 * If that's the case, we need to include the slideshow block manually.
+			 */
+			if ( ! class_exists( 'Automattic\Jetpack\Extensions\Slideshow' ) ) {
+				require_once JETPACK__PLUGIN_DIR . 'extensions/blocks/slideshow/slideshow.php';
+			}
+
 			return Slideshow\render_amp( $amp_args );
 		}