Added missing initialization of ConvexReactClient in Convex integration doc for React

Author: bikramswain1992

docs/guides/development/integrations/databases/convex.mdx

@@ -164,6 +164,12 @@ With [Convex](https://www.convex.dev/), you can build a backend with a provided
         throw new Error('Add your Clerk Publishable Key to the .env file')
       }
 
+      if (!process.env.PUBLIC_CONVEX_URL) {
+        throw new Error('Missing PUBLIC_CONVEX_URL in your .env file')
+      }
+
+      const convex = new ConvexReactClient(process.env.PUBLIC_CONVEX_URL)
+
       ReactDOM.createRoot(document.getElementById('root')!).render(
         <React.StrictMode>
           <ClerkProvider publishableKey={PUBLISHABLE_KEY} afterSignOutUrl="/">