If you follow the link I posted you will see that - according to the UN itself - UN Resolution 181 is "repealed" by UN Resolution 186. It's there in black and white (well, pixels, anyway). Also, as UNGA 181 was never accepted by the Security Council it is considered non-binding. The only way it could therefore be legally valid would be via the principle "pacta sunt servanda", i.e. if the disputing parties agreed to it, which they did not. Hence, it is neither valid nor binding, and is therefore totally irrelevant. Israel's right to exist does not rest upon it, but rather, as with the vast majority of states, "on assertion of independence by its people and government, on the vindication of that independence by arms against assault by other states, and on the establishment of orderly government within territory under its stable control." These are arguments of International Law and not just my personal opinion.
Militants loyal to Islamic State claim suicide bombings in Nigerian capital (Reuters)