Any particular reason you need react-native-fs? I used to have it in my NativeFileDocuments module but the orginal library is no longer maintained and the most active fork does not work well in the Mendix native template. I switched to react-native-blob-util. It has pretty much all the functionality of react-native-fs. Bonus is that it is already in the native template and the Make It Native app so you don't need custom builds. Make sure to keep the version in the widget the same as used in the Mendix native template.
Link below is for the Mendix 10.18 version of the template
https://github.com/mendix/native-template/blob/release/11.x.x/package.json