Why can I not use auth.currentuser.uid in getServerSideProps?

It may be a fundamental concept but I can't find anything to my issue.

const userDoc = getUserWithUID(auth.currentUser.uid);

Cannot read property 'uid' of null

import ImageUploader from "../components/ImageUploader";
import {auth, getUserWithUID, postToJSON} from "../lib/firebase";
import {useContext} from "react";
import {UserContext} from "../lib/context";

export async function getServerSideProps(context)  {
    const userDoc = getUserWithUID(auth.currentUser.uid);

    return {props: {userDoc}} ;
}

export default function SettingsPage() {
    return (
        <main>
            <ImageUploader></ImageUploader>
        </main>
    )

/ Firebase lib file

import firebase from 'firebase/app';
import 'firebase/auth';
import 'firebase/firestore';
import 'firebase/storage';

const firebaseConfig = {
    apiKey: process.env.NEXT_PUBLIC_API_KEY,
    authDomain: process.env.NEXT_PUBLIC_AUTH_DOMAIN,
    projectId: process.env.NEXT_PUBLIC_PROJECT_ID,
    storageBucket: process.env.NEXT_PUBLIC_STORAGE_BUCKET,
    messagingSenderId: process.env.NEXT_PUBLIC_MESSAGING_SENDER_ID,
    appId: process.env.NEXT_PUBLIC_APP_ID,
    measurementId: process.env.NEXT_PUBLIC_MEASUREMENT_ID
};

if(!firebase.apps.length) {
    firebase.initializeApp(firebaseConfig);
}

export const auth = firebase.auth();
export const googleAuthProvider = new firebase.auth.GoogleAuthProvider();

export const firestore = firebase.firestore();
// Storage exports
export const storage = firebase.storage();
export const STATE_CHANGED = firebase.storage.TaskEvent.STATE_CHANGED;

export const fromMillis = firebase.firestore.Timestamp.fromMillis;
export const serverTimestamp = firebase.firestore.FieldValue.serverTimestamp;

/**`
 * Gets a users/{uid} document with company
 * @param  {string} company
 */
export async function getUserWithCompany(company) {
    const usersRef = firestore.collection('users');
    const query = usersRef.where('company', '==', company).limit(1);
    const userDoc = (await query.get()).docs[0];
    return userDoc;
}

export async function getUserWithUID(uid) {
    const userDoc = await firestore.collection('users').doc(uid).get();
    return userDoc;
}