It would seem the error you are encountering has to do with the Ultra Dynamic Sky object not being valid / existing when you’re running your code. You can prevent the error by having a “Is Valid” node before getting the reference, or by using a “validated get”.
(Example of Validated Get vs. Is Valid)
This would however stop the code from running if it is not valid, and in case it is very important that you can access the component, then make sure it is valid / spawned in your world when running the code. Perhaps it is destroyed on End Play?