Widget.child throws if there is no child

[whitebyte]

auto widget = parent.childById("nonexistent"); // returns null
auto widget2 =parent.child(1000); // throws exception

Expected behavior: .child(x) should return null if there is no child with such index (or no children at all). I can open a PR if maintainers agree

[GrimMaple]

According to this the base class already returns null. I think that "return null" is intended behavior, so I don't mind such PR at all :) If you're at it -- please make sure that it's consistent across all subclasses