according to the documentation, Mobx, wants to get and call parameters in the form of inject in the sub-component, but there is a undefined problem?

// Timer.ts
interface IProps {
  timerStore?: TimerStore;
}

interface State {}

inject("timerStore")
@observer
export default class Timer extends React.Component<IProps, State> {
  constructor(props: IProps) {
    super(props);
    this.state = {};

    this.onClickBtn = this.onClickBtn.bind(this)
  }

  public onClickBtn():void {
    const tStore = this.props.timerStore as TimerStore;
    console.log(tStore);   // undefined
  }

  public render() {
    const tStore = this.props.timerStore as TimerStore;
    console.log(tStore); // undefined

    return (
      <div onClick={this.onClickBtn}>Seconds passed: {1}</div>
    )
  }
}

in App.ts , we can get the data normally, but when the sub-component Timer.ts injects store through inject , the corresponding store, will be undefined . After checking many reasons, we don"t know what the problem is. Ask the building to help solve it?