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It's strange that a method named getCell makes changes to the object.
What is the definition for that method?

Where is the collection modified while iterating over it? A reason for the error from the API doc:
If a single thread issues a sequence of method invocations that violates the contract of an object,...

Where does the code make any changes to the collection while it is looking at it with an iterator?

Can you post the full contents of the error message here?