Android: generated ReactNativeHostManager loads React Native twice and crashes (Feature flags cannot be overridden more than once) #492

The ReactNativeHostManager.kt that npm create @callstack/react-native-brownfield generates for a React Native CLI project crashes when the native app starts React Native:

java.lang.RuntimeException: Feature flags cannot be overridden more than once
  at com.facebook.react.internal.featureflags.ReactNativeFeatureFlags.override(ReactNativeFeatureFlags.kt:558)
  at com.facebook.react.defaults.DefaultNewArchitectureEntryPoint.load(DefaultNewArchitectureEntryPoint.kt:101)
  at com.callstack.reactnativebrownfield.ReactNativeBrownfield$Companion.loadNativeLibsInternal(ReactNativeBrownfield.kt:49)
  at com.callstack.reactnativebrownfield.ReactNativeBrownfield$Companion.initialize(ReactNativeBrownfield.kt:66)
  at com.rnapp.ReactNativeHostManager.initialize(ReactNativeHostManager.kt:28)

Cause: the template (ReactNativeHostManager.vanilla.kt) calls loadReactNative(application) and then ReactNativeBrownfield.initialize(application, reactHost, ...). Both run DefaultNewArchitectureEntryPoint.load(), which sets React Native's feature flags, so the second call throws. The nativeLibsLoaded guard in ReactNativeBrownfield.kt can't see the earlier loadReactNative call.

Repro: https://github.com/gronxb/react-native-brownfield-492-repro. It has the scaffold output and a minimal native Android app that starts React Native from a button. The README has the steps, and the comment below has videos of the crash and the fix.

Fix: remove loadReactNative(application) and pass the React host as a factory, as the Expo template (ReactNativeHostManager.post55.kt) already does:

ReactNativeBrownfield.initialize(application, onJSBundleLoaded) {
    getDefaultReactHost(
        context = application,
        packageList = PackageList(application).packages,
        jsMainModulePath = "index",
        jsBundleAssetPath = "index.android.bundle",
        jsBundleFilePath = null,
        useDevSupport = BuildConfig.DEBUG,
        jsRuntimeFactory = null
    )
}

Removing only loadReactNative is not enough. The template builds the host (val reactHost by lazy { getDefaultReactHost(...) }) as an argument to the deprecated initialize(application, reactHost, ...) overload, so it is built before initialize loads the native libraries, and the app then fails with SoLoader.init() not yet called. loadReactNative also turns on edge-to-edge when IS_EDGE_TO_EDGE_ENABLED is set, so a template fix should keep that part.

Tested with: @callstack/create-react-native-brownfield 5.3.0, @callstack/react-native-brownfield 5.3.0, React Native 0.85.2, Android emulator (API 37).

Edited: the first version of this issue suggested removing only loadReactNative, which isn't enough, as explained above.