我正在开发一个 React 应用，但我遇到了这个错误——“未捕获的类型错误：无法读取未定义的属性（读取‘长度’）”。如何修复这个问题？ 我的代码是：

import React from "react";
import { Grid, CircularProgress } from "@material-ui/core";
import Post from "./Post/Post";
import useStyles from "./styles";
import { useSelector } from "react-redux";

const Posts = () => {
  const posts = useSelector((state) => state.Posts);
  const classes = useStyles();

  console.log(posts);
  return (!posts.length ? <CircularProgress/> :(
   <Grid className={classes.container} container alignItems="stretch" spacing={3}>
      {
        posts.map((post)=> (
              <Grid key={post._id} item xs={12} sm={6}>
                <Post post = {post} />
              </Grid>
        ))
      }

   </Grid>
 )
  );
};

export default Posts;