When did treaties become void after a government change? and wouldn't this apply to all treaties at the same time? So if Iran is still a UN member even though it entered prior to the revolution why should other treaties become non-binding?
As for Israel: the non-proliferation treaty is still a voluntary measure originally designed as a a trade with civil technology being exchanged for forgoing military technology. There is no treaty most certainly not the UN charter forcing anyone to enter into the non-proliferation treaty, it does lack a get-out clause though.
There is much that can be criticized about Israel's politics and policies with quite valid grounds, including their threats against real or perceived enemies (Iran which routinely calls for attacks on Israel may not be the best example) - but Israel actually adhering to the UN charter and the non-proliferation treaty (as in choosing to not follow the recommendations of the General Assembly [and per the UN charter the GA cannot do more than issue recommendations] and as in choosing to not sign the treaty) is not really something you need to be an apologist for to point out as not being one of the valid causes of criticism.