Tips for fixing a server error when using NextJS and Sanity

export async function getServerSideProps() {
  const postData = await fetch('some/api/url')

  return {
    props: {
      post: postData || null
    },
  }
}

import { sanityClient } from "../../sanity";

const Exhibition = ({ title }) => {
  return <h1>{title}</h1>;
};

export const getServerSideProps = async (pageContext) => {
  const pageSlug = pageContext.query.slug;
  const query = `*[ _type == "exhibition" && slug.current == $pageSlug] [0]{
        title,
        artists[]{
            ...,
            artist->{
            name,
            slug,
            image,
            bio,
            }
        },
        mainImage,
        categories->{
            category,
        },
        startDate,
        endDate,
        images[]{
            ...,
        },
        text,
        guide,
        mediaAndDownloads,
    }`;

  const exhibition = await sanityClient.fetch(query, { pageSlug });

  if (!exhibition) {
    return {
      props: null,
      notFound: true,
    }
  } else {
    return {
      props: {
        title: exhibition.title,
        artists: exhibition.artists,
        mainImage: exhibition.mainImage,
        categories: exhibition.categories,
        startDate: exhibition.startDate,
        endDate: exhibition.endDate,
        images: exhibition.images,
        text: exhibition.text,
        guide: exhibition.guide,
        mediaAndDownloads: exhibition.mediaAndDownloads,
      },
    };
  }

};

export default Exhibition;