Is China's nuclear protection effective?

The nuclear umbrella provides Ukraine with nuclear protection, not a nuclear strike force.

In order to prevent the proliferation of nuclear weapons, the nuclear powers promised Ukraine: "If Ukraine is attacked by nuclear weapons, the nuclear powers will retaliate against the attacking country" in exchange for Ukraine giving up nuclear weapons and becoming a non-nuclear country.

Now Ukraine has become a recognized non-nuclear country.

The treaty is valid.

However, Russia does not need to use nuclear weapons to destroy Ukraine at all, so it has nothing to do with the treaty.