The phrase "render null and void" essentially means to cancel, invalidate or annul something. The antonym of this phrase would be something that verifies, approves or confirms. Examples of antonyms for this phrase could include validate, authorize, substantiate, or endorse. These words suggest the opposite intent of nullifying or voiding something, and instead approve or affirm it. Other antonyms could include legalize, legitimize or uphold. These words emphasize the opposite meaning of rendering something null and void, and instead, placing it in a state of acceptance or validation.