Responding to slash command in socket mode using node v20 causes exception

[filmaj]

When running an app in socket mode, have the app create a modal with an e.g. plain text input with an action_id using a slash command. For example, register a slash command that opens a modal:

app.command('/post', async ({ ack, body, client, logger }) => {
  console.log('post coming in');
  // Acknowledge the command request
  await ack();
});

Then trigger the slash command (/post in the above example), you will see output like:

[DEBUG]  bolt-app initialized
post coming in
[DEBUG]  socket-mode:SocketModeClient:0 Calling ack() - type: slash_commands, envelope_id: 44639bd2-7246-47cc-a587-80b6c3276cde, data: undefined
[DEBUG]  socket-mode:SocketModeClient:0 send() method was called in state: connected, state hierarchy: connected,ready
[DEBUG]  socket-mode:SocketModeClient:0 Sending a WebSocket message: {"envelope_id":"44639bd2-7246-47cc-a587-80b6c3276cde","payload":{}}
/Users/fmaj/src/slapp-BoltJSApp/node_modules/@slack/socket-mode/dist/SocketModeClient.js:320
                        this.logger.error(`Failed to send a WebSocket message (error: ${error.message})`);
                                                                                              ^

TypeError: Cannot read properties of null (reading 'message')
    at Object.callback (/Users/fmaj/src/slapp-BoltJSApp/node_modules/@slack/socket-mode/dist/SocketModeClient.js:320:95)
    at callback (node:internal/streams/writable:562:21)
    at afterWrite (node:internal/streams/writable:507:5)
    at onwrite (node:internal/streams/writable:487:7)
    at WriteWrap.onWriteComplete [as oncomplete] (node:internal/stream_base_commons:106:10)

Node.js v20.5.0

This does not happen in node v18.

Reproducible in:

The Slack SDK version

3.12.3

Node.js runtime version

v20.5.0

[filmaj]

I noticed this commit in the node-slack-sdk: slackapi/node-slack-sdk@1c72709

I think the same fix needs to be applied in all of our libraries.

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[github-actions]

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