There was a problem processing the Slack event MESSAGE_EVENT ErrorType: NullPointerException

Hi Maksimilian, that exact wording — "There was a problem processing the Slack event MESSAGE_EVENT ErrorType: NullPointerException" — is what the Apex SDK for Slack runtime posts as an ephemeral message when the Apex handler you registered for the message event throws an unhandled exception, so the NullPointerException is happening inside your own message-event handler class, not on Slack's side. It fires on every message because your app is subscribed to the message event (e.g. message.channels) and the handler runs for each one; it does not fire when you send from Salesforce because that path never invokes the inbound message handler. The usual culprit is a message subtype that doesn't carry the field you're reading — bot_message (including your app's own echoed messages), message_changed/message_deleted, and join/leave events have no user and often no text, so event.getUser()/getText() or a downstream SOQL lookup that finds nothing comes back null. Fix it in the handler: early-return when a subtype is present or when user/text is null (ignore bot and echoed messages), null-check any record you query, and wrap the body in try/catch with a debug log so you can see the exact line. If you need the noise to stop immediately, remove the message event from Event Subscriptions in the app manifest, then re-add it once the handler is guarded. If this helps, please mark it as the Best Answer so it helps the next person — thanks 🙂