rename file and do dom element check

src/main.ts

@@ -1,8 +1,14 @@
-import './styles.postcss';
+import './styles.pcss';
 import App from './App.svelte';
 
+const target = document.getElementById('app');
+if (!target)
+  throw new Error(
+    "The element with id of 'app' wasn't found on the base html file."
+  );
+
 const app = new App({
-  target: document.getElementById('app'),
+  target,
   intro: true
 });