docs: added a fix for QueryClient error in medusa-react upgrade guide (#3089)

docs/content/advanced/backend/upgrade-guides/medusa-react/4-0-2.md

@@ -52,3 +52,36 @@ import { QueryClient } from "@tanstack/react-query"
 // this remains the same
 const queryClient = new QueryClient()
 ```
+
+### Fix No QueryClient set Errors
+
+If you're using a Next.js storefront, you might face the following error after this update when you run your storefront:
+
+```bash
+No QueryClient set
+```
+
+This is due to an issue related to Tanstack Query shipping `esm` modules in its latest versions and how Next.js uses Webpack.
+
+Although in future versions of `medusa-react` this issue will be fixed, you can add the following into `next.config.js` to fix this error:
+
+```js
+const path = require("path")
+
+/** @type {import('next').NextConfig} */
+const nextConfig = {
+  // ... other configs
+  webpack: (config, options) => {
+    if (options.isServer) {
+      config.externals = ["@tanstack/react-query", ...config.externals]
+    }
+    const reactQuery = path.resolve(
+      require.resolve("@tanstack/react-query")
+    )
+    config.resolve.alias["@tanstack/react-query"] = reactQuery
+    return config
+  },
+}
+
+module.exports = nextConfig
+```