MMKV is undefined on iOS

It not always the case, in most cases our app startup normal.

But around 10% of which, the startup will crash directly because MMKV is undefined. and when it happens it require an app kill and restart to resume functioning.

We are using react-native 0.64

```
import { MMKV } from 'react-native-mmkv';
MMKV.getString(key)
```

```
"react-native-mmkv": "^1.1.6",
```