error on incorrect init args

src/middleware.ts

@@ -54,6 +54,10 @@ export default function middleware({ App, routes, store }: {
 	routes: RouteObject[],
 	store: (req: Req) => Store
 }) {
+	if (!App) {
+		throw new Error(`As of 0.12, you must supply an App component to Sapper — see https://sapper.svelte.technology/guide#0-11-to-0-12 for more information`);
+	}
+
 	const output = locations.dest();
 
 	const client_info = JSON.parse(fs.readFileSync(path.join(output, 'client_info.json'), 'utf-8'));

src/runtime/index.ts

@@ -251,6 +251,10 @@ function trigger_prefetch(event: MouseEvent | TouchEvent) {
 let inited: boolean;
 
 export function init(opts: { App: ComponentConstructor, target: Node, routes: Route[], store?: (data: any) => Store }) {
+	if (opts instanceof HTMLElement) {
+		throw new Error(`The signature of init(...) has changed — see https://sapper.svelte.technology/guide#0-11-to-0-12 for more information`);
+	}
+
 	App = opts.App;
 	target = opts.target;
 	routes = opts.routes.filter(r => !r.error);