fix(examples): guard missing #app mount target in svelte simple example (#10144)

Author: grzdev

examples/svelte/simple/src/main.ts

@@ -2,8 +2,9 @@ import { mount } from 'svelte'
 import './app.css'
 import App from './App.svelte'
 
-const app = mount(App, {
-  target: document.querySelector('#app')!,
-})
+const target = document.querySelector('#app')
+if (!target) throw new Error('Missing #app element')
+
+const app = mount(App, { target })
 
 export default app