[Feature request] BaseBoxyDelegate.getChildOrNull

[jtarkowski27]

For convenience sake, I'd like to suggest adding a simple non-throwable method getChildOrNull to BaseBoxyDelegate. With that it wouldn't be necessary to always write code like this:

BoxyChild? child = null;
if (hasChild(#child)) {
  child = getChild(#child);
}

Implementation is simple and would go as follows:

/// Gets the child handle with the specified [id] or returns null if there
/// is no child with given [id].
T? getChildOrNull<T extends ChildHandleType>(Object id) {
  if (hasChild(id)) {
    return getChild(id);
  }
  return null;
}

[pingbird]

This is reasonable, feel free to submit a PR!