Execution failed for task ':app:compileDebugKotlin'. React native update

I've just updated React Native from 0.71.11 to 0.73.7 following the instructions from the React Native upgrade helper. When I try to run 'yarn android', I get this error

error: android/app/src/main/java/com/useapp/MainApplication.kt:18:41 Return type of 'getPackages' is not a subtype of the return type of the overridden member 'protected/protected and package/ abstract fun getPackages(): (MutableList<ReactPackage!>..kotlin.collections.List<ReactPackage!>?) defined in com.facebook.react.defaults.DefaultReactNativeHost' e: /android/app/src/main/java/com/useapp/MainApplication.kt:19:24 Type mismatch: inferred type is ArrayList<ReactPackage!>! but List was expected e: android/app/src/main/java/com/useapp/MainApplication.kt:35:13 Unresolved reference: ReactHost

FAILURE: Build failed with an exception.

• What went wrong: Execution failed for task ':app:compileDebugKotlin'.

A failure occurred while executing org.jetbrains.kotlin.compilerRunner.GradleCompilerRunnerWithWorkers$GradleKotlinCompilerWorkAction Compilation error. See log for more details

MainApplication.kt

package xxx.xxx

import android.app.Application
import com.facebook.react.PackageList
import com.facebook.react.ReactApplication
import com.facebook.react.ReactNativeHost
import com.facebook.react.ReactPackage
import com.facebook.react.defaults.DefaultReactNativeHost
import com.facebook.soloader.SoLoader
import java.util.List
import com.vinzscam.reactnativefileviewer.RNFileViewerPackage
import com.oblador.vectoricons.VectorIconsPackage

class MainApplication : Application(), ReactApplication {

    override val reactNativeHost: ReactNativeHost =
        object : DefaultReactNativeHost(this) {
            override fun getPackages(): List<ReactPackage> {
                return PackageList(this).packages // Use PackageList to get packages
            }

            override fun getJSMainModuleName(): String = "index"

            override fun getUseDeveloperSupport(): Boolean = BuildConfig.DEBUG

            override val isNewArchEnabled: Boolean = BuildConfig.IS_NEW_ARCHITECTURE_ENABLED
            override val isHermesEnabled: Boolean = BuildConfig.IS_HERMES_ENABLED
        }

    override fun onCreate() {
        super.onCreate()
        SoLoader.init(this, false)
        if (BuildConfig.IS_NEW_ARCHITECTURE_ENABLED) {
            // If you opted-in for the New Architecture, we load the native entry point for this app.
            ReactHost.load() // This seems to be a custom function, make sure it's defined correctly
        }
        // ReactNativeFlipper.initializeFlipper(this, reactNativeHost.reactInstanceManager)
        // This line is commented out as ReactNativeFlipper seems unresolved in your code
    }
}

MainActivity.kt

package xxx.xxx
import com.facebook.react.ReactActivity
import com.facebook.react.ReactActivityDelegate
import com.facebook.react.defaults.DefaultReactActivityDelegate
import android.os.Bundle

class MainActivity : ReactActivity() {

    /**
     * Returns the name of the main component registered from JavaScript. This is used to schedule
     * rendering of the component.
     */
    override fun getMainComponentName(): String = "xxx"

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }

    /**
     * Returns the instance of the [ReactActivityDelegate]. We use [DefaultReactActivityDelegate]
     * which allows you to enable New Architecture with a single boolean flags [fabricEnabled]
     */
    override fun createReactActivityDelegate(): ReactActivityDelegate =
        DefaultReactActivityDelegate(this, "useapp") // Provide mainComponentName as a string
}

I've tried to mess with both files, but so far without success.