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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Before using length, check the input is really an array. Check MDN Docs
Not all input values are arrays (or strings) that have length property. In your case the passed value was null (it's there in the error message).