我想我可能在useRef这里滥用了。当我尝试画到画布上，我得到以下错误：cannot set 'fillStyle' of undefined。

import React, { useEffect, useRef } from "react";
import useWindowSize from "../hooks/useWindowSize";

export default function Player() {
  const canvasRef = useRef();
  const ctx = useRef();

  useEffect(() => {
    ctx.current = canvasRef.current.getContext("2d");
  }, []);
  ctx.current.fillStyle = "green";
  ctx.current.fillRect(20, 20, 150, 100);
  return (
    <React.Fragment>
      <div>Hello</div>
      <canvas ref={canvasRef} width="500" height="500" />
    </React.Fragment>
  );
}