[JavaKit] `@JavaImplementation` method returning a null instance from Swift to Java

I'm trying to wrap RecyclerView.Adapter in Android and create the row view from Swift and pass it to Java via JNI. Unfortunately on the Java / Kotlin side the returned value is always null.

RecyclerViewAdapter.kt

// Create new views (invoked by the layout manager)
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerViewAdapter.ViewHolder {
        val viewHolder = onCreateViewHolderSwift(parent, viewType)
        checkNotNull(viewHolder)
        checkNotNull(viewHolder.itemView)
        return viewHolder
    }

RecyclerViewAdapter.swift

@JavaImplementation("com.pureswift.swiftandroid.RecyclerViewAdapter")
extension RecyclerViewAdapter {
    
    @JavaMethod
    public func onCreateViewHolderSwift(_ viewGroup: ViewGroup?, _ viewType: Int32) -> RecyclerViewAdapter.ViewHolder? {
        log("\(self).\(#function) \(viewType)")
        let viewHolder = callback.onCreateViewHolder(viewGroup!, viewType)
        log("\(self).\(#function) Created \(viewHolder.getClass().getName()) \(viewHolder.itemView.getClass().getName())")
        return viewHolder
    }

I've verified from the Swift side that the Java Object I'm returning is a valid instance, and print their Java class name. But the checkNotNull() still fails on the JVM side.

16:57:54.394  D  SwiftAndroidApp.RecyclerViewAdapter.onCreateViewHolderSwift(_:_:) Created com.pureswift.swiftandroid.RecyclerViewAdapter$ViewHolder android.widget.TextView
16:57:54.394  D  Shutting down VM
16:57:54.395  E  FATAL EXCEPTION: main
Process: com.pureswift.swiftandroid, PID: 7551
java.lang.IllegalStateException: Required value was null.

Android.zip

android-crash.log

Reproduce the failure using the attached Android.zip sample and android-crash.log. Start with the @JavaMethod in RecyclerViewAdapter.swift and its Kotlin caller in RecyclerViewAdapter.kt, then trace the Swift-to-Java return boundary. Done means the returned ViewHolder reaches the JVM as non-null and the checkNotNull failure no longer occurs.