ahora voy a por el título

src/routes/[slug]/+page.svelte

@@ -5,7 +5,7 @@
 	import Preview from '$lib/preview/Preview.svelte';
 
 	const slug = data.slug;
-	const templateIndex = templates.map((e) => e.name).indexOf(slug);
+	/** @type {number|undefined} */ const templateIndex = templates.map((e) => e.name).indexOf(slug);
 </script>
 
 <svelte:head>
@@ -18,7 +18,11 @@
 
 <div class="main">
 	<section class="form"><Form /></section>
-	<section class="preview"><Preview {templateIndex} /></section>
+	{#if templateIndex !== undefined}
+		<section class="preview">
+			<Preview {templateIndex} />
+		</section>
+	{/if}
 </div>
 
 <style>