[RAM] Unhandled Exception When Creating Slack Connector

[jcger]

Description:

When interacting with the Create Slack Connector modal, a critical bug has been identified that causes an application failure. The bug manifests when switching back and forth between the web api and webhook options. See screenrecording

Error: Cannot read properties of undefined (reading 'length')
    at useFetchChannels 

https://github.com/elastic/kibana/assets/17549662/5f79ae31-324e-4faa-848c-441b349c157e

Steps to Reproduce:

• Launch the Slack Connector API.
• Click on the 'web api' option.
• Without performing any other actions, click on the 'webhook' option.
• Again, switch back to the 'web api' option.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)