这是5-9的代码，我专门仿写了一段代码测试了一下

class A extends _B {
  B? b;
  A._();
  static A? _instance;
  static A getInstance() {
    if (_instance == null) {
      return A._();
    }

    return _instance!;
  }

  void registerB(B b) {
    LogUtil.e(b, tag: "A test");
    this.b = b;
    LogUtil.e(this.b, tag: "A test");
  }

  @override
  void onJumpTo() {
    b?.onJumpTo();
  }
}

abstract class _B {
  void onJumpTo();
}

class B {
  final VoidCallback onJumpTo;
  B({required this.onJumpTo});
}

为什么registerB不成功呢，调用代码如下：

A.getInstance().registerB(B(onJumpTo: () {}));
LogUtil.e(A.getInstance().b, tag: "test");

明明前面有调用registerB，结果A中成员b仍是null
打印结果如下：可见registerB没有成功

I/flutter (28335): A test e  Instance of 'B'
I/flutter (28335): A test e  Instance of 'B'
I/flutter (28335): test e  null

老师看下我的代码哪里面有问题