# Render Error: Cannot read property 'createClient' of null

> Faced some issues tinkering with React Native's BLE library and trying to get it to work.

By Tang Yetong. Published November 5, 2023. 3 min read. Category: mobile. Tags: React Native, BLE, Expo, Debugging.
Read it with the interactive demos: https://tangyetong.com/blog/react-native-ble-createclient-error

Faced some issues tinkering with React Native's BLE library and trying to get it to work. Was following this [repo](https://github.com/friyiajr/BLESampleExpo) and was faced with `Render Error: Cannot read property 'createClient' of null` while trying to load the app on Expo Go.

The app seems to be crashing on the line: `new BleManager()`, but loading the app on emulator works fine and crashes on Expo Go. Weird!

## Development Server

1) After playing around I finally got `BleManager()` to stop crashing after setting up the project to use a development build by running 

```bash
expo prebuild
```

2) Create a development build like so:

```bash
npx eas build --profile development
```

3) Scan the QR, install the app on your phone and open it. Then you will need to run the dev server for your development build:

```bash
yarn start --dev-client
```

The server should appear on the development build on your phone and then you can run your mobile app.