React ref.current is null

So the thing about refs is that they aren’t guaranteed to be set on first render. You can be sure they are set during and after componentDidMount so you have two ways going forward.

You could use the callback style ref and set state based on that. E.g. instead of passing your ref as the prop, you can pass in a reference to a function like this.handleRef and it would do some logic in there:

  handleRef = r => {
    this.setState({ bodyHeight: r.clientHeight})
    this.calendarBodyRef.current = r;
  };

Or, you could keep your current set up but you would have to move your clientHeight bit to a lifecycle function like:

  componentDidMount() {
    this.setState({ bodyHeight: this.calendarBodyRef.current.clientHeight });
  }

Ultimately, you can’t immediately read the current value of a ref like that, you would have to check it after the render and then read your bodyHeight from state.