Why am I getting a java.lang.ClassCastException in Kotlin?

I wrote this sample code that I extracted from a library that I'm making because it is causing a java.lang.ClassCastException and I don't understand why. Here's the code:

interface Event

data class Update(val message: Message?)
open class Message(open val text: String?)
data class TextMessage(override val text: String) : Message(text = text)

class MessageReceiveEvent(
    val message: Message,
) : Event

class TextMessageReceiveEvent(
    val message: TextMessage,
) : Event

fun handleUpdate(update: Update): Event {
    if (update.message != null) {
        if (update.message.text != null) return TextMessageReceiveEvent(update.message as TextMessage)
        return MessageReceiveEvent(update.message)
    }
    error("Update message is null.")
}

handleUpdate(
    Update(
        message = Message(
            text = "Text" // if this is null, the program successfully runs.
        )
    )
).let {
    if (it is MessageReceiveEvent) println("Message without text.")
    if (it is TextMessageReceiveEvent) println("Message with text.")
}

Copyright License：
Author:「Dj Sushi」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/72953632/why-am-i-getting-a-java-lang-classcastexception-in-kotlin