Unable to connect to server with synchronous event handling

[fvoichick]

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Make sure you've done the following:

• You're using the latest build for your server version
• This isn't an issue caused by another plugin
• You've checked for duplicate issues
• You didn't use /reload

Debug paste link:
https://pastebin.com/TPFuRzU9 (console error message)
https://pastebin.com/Ju8N1Xqs (protocol dump)

Description and relevant errors:
I created a plugin with the following (Kotlin) code:

package com.voichick.cyclic

import com.comphenix.protocol.PacketType
import com.comphenix.protocol.ProtocolLibrary
import com.comphenix.protocol.events.PacketAdapter
import com.comphenix.protocol.events.PacketEvent
import org.bukkit.plugin.java.JavaPlugin

@Suppress("unused")
class Cyclic : JavaPlugin() {
    override fun onEnable() {
        val listener = object : PacketAdapter(this, PacketType.Play.Server.POSITION) {
            override fun onPacketSending(event: PacketEvent?) {
                logger.info("Sending packet!")
            }
        }
        val manager = ProtocolLibrary.getProtocolManager()
        manager.asynchronousManager.registerAsyncHandler(listener).syncStart()
    }
}

Now, whenever I try to log into the server, there's an error and I'm unable to.

When I make the following replacement:

//      manager.asynchronousManager.registerAsyncHandler(listener).syncStart()
        manager.addPacketListener(listener)

...it works as expected.
My understanding is that the two lines above should be fairly equivalent, except that the first one is run synchronously and the second is run asynchronously, but I'm not sure if that's correct. Regardless, I don't think that the code above should prevent players from joining the server.