Speaking of entities that no longer exist, try the government of Ukraine that died in a coup in 2014. Whatever agreement was made with Ukraine died in the coup.
Ok, so the validity of international agreements is now dependent upon the same government still being in power? Or is it that the country must be either a democracy or republic? Not sure you want to go down that road to defend Russia.
Ukraine had a politic crisis between a parliament that had voted almost unanimously for closer economic ties with Europe, versus a President who had campaigned on agreeing with that but then sought to reverse course at the last moment to join the Russian economic group instead. Regardless of who was right, it was the right of the Ukrainian people, not the Russian government, to resolve that. The Budapest Agreement made that absolutely clear.