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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.
And dont hesitate to ask, if you have questions. :-)
First of all: Do not create an issue, if you do not know, why you get an exception/error. Issues are for known/proven problems.
The problem in your code is the way you checking for null.
Check it line for line. I think you will find the problem. Otherwise ask again and I will show you. :-)