报错：The following assertion was thrown while handling a gesture:
invalid text selection: TextSelection(baseOffset: 2, extentOffset: 2, affinity: TextAffinity.downstream, isDirectional: false)
解决办法：
将searchController.clear(); 替换成如下
onCancel() {
// 保证在组件build的第一帧时才去触发取消清空内容
WidgetsBinding.instance.addPostFrameCallback((_) => searchController.clear.clear());
}
