Try fixing some type errors

Author: captbaritone

packages/webamp/demo/js/index.tsx

@@ -159,7 +159,11 @@ async function main() {
     if (backgroundColor != null) {
       window.document.body.style.backgroundColor = backgroundColor;
     }
-    const root = ReactDOM.createRoot(document.getElementById("demo-desktop"));
+    const div = document.getElementById("demo-desktop");
+    if (div == null) {
+      throw new Error("Could not locate #demo-desktop div");
+    }
+    const root = ReactDOM.createRoot(div);
     root.render(
       <DemoDesktop webamp={webamp} soundCloudPlaylist={soundcloudPlaylist} />
     );

packages/webamp/tsconfig.json

@@ -10,7 +10,8 @@
     "strict": true,
     "jsx": "preserve",
     "noEmit": true,
-    "pretty": true
+    "pretty": true,
+    "types": ["react/experimental", "react-dom/experimental"]
   },
   "include": ["**/*.ts", "**/*.tsx"],
   "exclude": ["node_modules", "demo/built"]