It's possible to create an experiment that won't let you create a cohort via API.

[aldenhallak]

Unhandled error TypeError: Cannot read properties of undefined (reading 'length')
    at createChatStagePublicData (/workspace/lib/index.js:56749:54)
    at /workspace/lib/index.js:58249:29
    at Array.forEach (<anonymous>)
    at createPublicDataFromStageConfigs2 (/workspace/lib/index.js:58246:14)
    at createCohortInternal (/workspace/lib/index.js:67353:74)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async /workspace/lib/index.js:67421:5
    at async Transaction.runTransactionOnce (/workspace/node_modules/@google-cloud/firestore/build/src/transaction.js:440:28)
    at async /workspace/node_modules/@google-cloud/firestore/build/src/transaction.js:412:28
    at async /workspace/lib/index.js:67420:3

If discussions isn't passed in to ChatStageConfig, it will succeed but then you won't be able to create a cohort.