Getting Error while logging into homeserver using react native

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### Getting this error when using `matrixClient.startClient();` in react native:
```
Getting push rules failed, [ReferenceError: Can't find variable: document]
- node_modules\react-native\Libraries\YellowBox\YellowBox.js:63:8 in console.error
- node_modules\expo\build\environment\muteWarnings.fx.js:27:24 in error
- node_modules\matrix-js-sdk\lib\sync.js:505:2 in getPushRules
- node_modules\regenerator-runtime\runtime.js:45:44 in tryCatch
- node_modules\regenerator-runtime\runtime.js:274:30 in invoke
- node_modules\regenerator-runtime\runtime.js:45:44 in tryCatch
- node_modules\regenerator-runtime\runtime.js:135:28 in invoke
- node_modules\regenerator-runtime\runtime.js:147:19 in PromiseImpl.resolve.then$argument_1
- node_modules\promise\setimmediate\core.js:37:14 in tryCallOne
- node_modules\promise\setimmediate\core.js:123:25 in setImmediate$argument_0
- node_modules\react-native\Libraries\Core\Timers\JSTimers.js:146:14 in _callTimer
- node_modules\react-native\Libraries\Core\Timers\JSTimers.js:194:17 in _callImmediatesPass
- node_modules\react-native\Libraries\Core\Timers\JSTimers.js:458:30 in callImmediates
* [native code]:null in callImmediates
- node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:407:6 in __callImmediates
- node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:143:6 in __guard$argument_0
- node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:384:10 in __guard
- node_modules\react-native\Libraries\BatchedBridge\MessageQueue.js:142:17 in __guard$argument_0
* [native code]:null in flushedQueue
* [native code]:null in callFunctionReturnFlushedQueue
```
### My code is:
```
const myUserId = "@chakerthehacker:matrix.org";
const myAccessToken = "im not leaking my token";
const matrixClient = sdk.createClient({
baseUrl: "https://matrix-client.matrix.org",
accessToken: myAccessToken,
userId: myUserId
});
matrixClient.startClient();
```

Start with matrixClient.startClient() and the stack trace location node_modules/matrix-js-sdk/lib/sync.js:505, then trace the getPushRules call in a React Native environment. Done means startup no longer reports the document ReferenceError and the client can proceed through login and sync.

Written by the indexing model from the issue text.