I'm trying to follow Enabling threaded Slack messages while using the Jenkins Slack plugin, and have created the app, and sent it to my Slack workspace. The xoxp-*** is from the "OAuth & Permissions" menu of the app I created that has Bot chat:write permissions, that I've installed to my workspace. To test I do this

node("build-node") {
    final String slackUrl = "***"
    final String token = "xoxp-***"
    stage("Test") {
        def slackResponse = slackSend(botUser: true, baseUrl: slackUrl, tokenCredentialId: token, channel: "ecs-feature", message: "Here is the primary message")
        slackSend(botUser: true, channel: slackResponse.threadId, message: "Thread reply #1")
        slackSend(botUser: true, channel: slackResponse.threadId, message: "Thread reply #2")
    }
}

but get:

Slack Send Pipeline step running, values are - baseUrl: ***, teamDomain: myWorkspace, channel: ecs-feature, color: <empty>, botUser: true, tokenCredentialId: xoxp-***, notifyCommitters: false, iconEmoji: <empty>, username: <empty>, timestamp: <empty>
ERROR: Slack notification failed with exception: java.lang.IllegalArgumentException: the credential with the provided ID (xoxp-***) could not be found and no token was specified
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Also:   .jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: b8ca58a3-c052-47e0-b524-5dee1c6913d0
java.lang.NullPointerException: Cannot get property 'threadId' on null object
    at .codehaus.groovy.runtime.NullObject.getProperty(NullObject.java:60)
    at .codehaus.groovy.runtime.InvokerHelper.getProperty(InvokerHelper.java:190)
    at .codehaus.groovy.runtime.ScriptBytecodeAdapter.getProperty(ScriptBytecodeAdapter.java:469)

Any clues?