https://github.com/CRAnimation/CRBoxInputView/blob/5e45530ed234ceffd4f7b2b1b2c5a2b08b2a7ed8/PodCode/Classes/CRBoxInputView.h#L163 实现并未看到 `return nil` 的处理，为何返回实例标记为 `_Nullable`
CRBoxInputView/PodCode/Classes/CRBoxInputView.h
Line 163 in 5e45530
实现并未看到
return nil的处理，为何返回实例标记为_Nullable