How to fix this issue Assertion filed

Question

How to fix this issue. Someone can help me?

My code

@override
  void forgetChild(Element child) {
    assert(child == _child);
    _child = null;
    super.forgetChild(child);
  }

This is my Element class code.

Element? _child;

  bool _debugDoingBuild = false;
  @override
  bool get debugDoingBuild => _debugDoingBuild;

  @override
  void mount(Element? parent, Object? newSlot) {
    super.mount(parent, newSlot);
    assert(_child == null);
    assert(_lifecycleState == _ElementLifecycle.active);
    _firstBuild();
    assert(_child != null);
  }