Resolve Sentry issues

- Add the missing not found check in artifact admin
- Add null-checks in TierList's global error handler

app/(ui)/artifact/admin/[id]/page.tsx

@@ -1,4 +1,5 @@
 import { eq } from "drizzle-orm";
+import { notFound } from "next/navigation";
 import Avatar from "@/components/avatar";
 import {
   Card,
@@ -23,6 +24,7 @@ export default async function AdminSubmissionView({
     .from(submissions)
     .where(eq(submissions.id, id))
     .limit(1);
+  if (!sub) notFound();
   const [char] = await db
     .select()
     .from(characters)

app/(ui)/tl/[type]/[ver]/tierlist.tsx

@@ -129,7 +129,7 @@ export function TierList({
   useEffect(() => {
     const handler = (ev: ErrorEvent) =>
       toast.error("Unexpected Error Occured", {
-        description: `${ev.error.message || ev.error || "*unknown error*"}`,
+        description: `${ev?.error?.message || ev?.error || "*unknown error*"}`,
       });
     window.addEventListener("error", handler);
     return () => window.removeEventListener("error", handler);