A post cannot be linked with the navigation menu. For that we use page. So this component will always reply on the slug to render a post. For this we will use the data connector SinglePostData.js to wrap this component.

Below are the props that will be passed on to this component.

{

    "match": {
      "path": "/post/:slug",
      "url": "/post/hello-world",
      "isExact": true,
      "params": {
        "slug": "hello-world
      }
    },
    "location": {...},
    "history": {...},
    "settings": {...},
    "post": {... },
    "loading": false #true/false
  }

You can handle an invalid slug by checking if post is null.

To show adjacent posts, you can wrap this component with AdjacentPostsData.js to get additional props adjacentPosts and adjPostsLoading.
The data in adjacentPosts will have previous and next.

adjacentPosts = {previous: {}, next:{}} ;

This is an example component of how your code may look like.

import React, { Component } from "react";
import PropTypes from "prop-types";
import Article from "../components/Post/Article";
import Loader from "../components/Loader";
import OhSnap from "client/helpers/OhSnap";
import SinglePostData from "shared/data-connectors/SinglePostData";

class SinglePost extends Component {
    render() {
        if (this.props.loading) {
            return <Loader />;
        }
        if (this.props.post === null) {
            return (
                <OhSnap message="Sorry, this page does not exist or might be restricted." />
            );
        }
        return (
            <Article post={ this.props.post} />
        );
    }
}

SinglePost.propTypes = {
    page: PropTypes.object,
    loading: PropTypes.bool
};

export default SinglePostData(SinglePost);