Subscriptions: Prevent warning when $post global is malformed (#48074)

Author: tbradsha

projects/plugins/jetpack/changelog/fix-subscriptions-address_warnings_in_the_wild

@@ -0,0 +1,4 @@
+Significance: patch
+Type: other
+
+Subscriptions: Prevent warning when $post global is malformed.

projects/plugins/jetpack/modules/subscriptions/subscribe-modal/class-jetpack-subscribe-modal.php

@@ -233,8 +233,11 @@ public function should_user_see_modal() {
 			return false;
 		}
 
-		// Don't show if post is for subscribers only or has paywall block
+		// Don't show if post is not a post, is for subscribers only, or has paywall block
 		global $post;
+		if ( ! $post instanceof WP_Post ) {
+			return false;
+		}
 		if ( defined( 'Automattic\\Jetpack\\Extensions\\Subscriptions\\META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS' ) ) {
 			$access_level = get_post_meta( $post->ID, META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS, true );
 		} else {