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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.
No answer after 6 months, closing.
You're probably not testing if it is an array before trying to get some property or applying some array's method. But I can only guess without seeing your code. If you post it, use proper markdown and mark your post as having spoiler content.