What is meant by the requirement that 'GameObject' type should not be a nullable value type?

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This means that a GameObject cannot be set to null, or have a null value. A GameObject is a reference type in Unity, meaning that it is a variable that holds a reference to an instance of an object. Reference types can be null, but Unity's GameObject type cannot be null because it is a value type. Therefore, attempting to set a GameObject to null will result in an error.