Gracefully handle plugin responses

Not every ID maps to an existing page, so in this case the URL is dynamically constructed.

Fixes #20.

src/StatiCache.php

@@ -4,6 +4,7 @@
 
 use Closure;
 use Kirby\Cms\App;
+use Kirby\Cms\Url;
 use Kirby\Filesystem\F;
 use Kirby\Filesystem\Mime;
 use Kirby\Toolkit\Str;
@@ -104,7 +105,7 @@ protected function file(string|array $key): string
 		}
 
 		$page = $kirby->page($key['id']);
-		$url  = $page->url($key['language']);
+		$url  = $page?->url($key['language']) ?? Url::to($key['id']);
 
 		// content representation paths of the home page contain the home slug
 		if ($page->isHomePage() === true && $key['contentType'] !== 'html') {