How to check if object is valid?

[Maris]

Let's say I have a variable:

obj = nil;

Let's say I initialized it somewhere:

for i = 0, worldObjects:size() - 1 do
	obj = worldObjects:get(i);
	break;
end

Then some time passed. The object could be removed from the world.

How to check that the object referenced by a variable still exists?