The idea of declaring something null and void can have various antonyms depending on the context. In legal terms, the antonym might be "valid" or "enforceable." For example, a contract that is not null and void would be considered valid and enforceable. In a more general sense, antonyms might include "agreed upon," "approved," or "ordered." For example, a decision that is not null and void might be considered an agreed-upon decision or an ordered decision. Additionally, antonyms such as "confirmed," "ratified," or "validated" might be appropriate depending on the situation. Ultimately, the antonym for declaring something null and void will depend on the context and the specific circumstances involved.