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在尝试Dart的Sound Null Safety 时，我遇到了一个问题：

一些上下文

创建一个新的Flutter项目我发现以下（并且非常熟悉）片段代码

int _counter = 0;

void _incrementCounter() {
  setState(() {
    // This call to setState tells the Flutter framework that something has
    // changed in this State, which causes it to rerun the build method below
    // so that the display can reflect the updated values. If we changed
    // _counter without calling setState(), then the build method would not be
    // called again, and so nothing would appear …