Why do I get an Exception when pushing objects to Stack?

Question

I've been debugging my solution to this LeetCode problem about Twitter Design. I'm still debugging the code, but my main concern is regarding the NullPointerException I get when I try to push Integers to a Stack in a static HashMap.

Here's my code:

import java.util.Stack;
import java.util.HashMap;
import java.util.HashSet;
public class Twitter {

private static HashMap<Integer, Stack<Integer>> newsFeed;
private HashMap<Integer, HashSet<Integer>> followList;
/** Initialize your data structure here. */
public Twitter() {
    //Holds a userId as a key, with a Stack of tweets acting as a newfeed.
    newsFeed = new HashMap<Integer, Stack<Integer>>();
    //Holds a userId as a key, with a HashSet of userIds in the user's follow list
    followList = new HashMap<Integer, HashSet<Integer>>();
}

/** Compose a new tweet. */
public void postTweet(int userId, int tweetId) {
    /** When a user tweets, the tweet is seen by the user and all the user's followers; therefore, each tweet posted requires an update to the newsfeed Stack of the user and all their followers. */
    Stack<Integer> userFeed = newsFeed.get(userId);
    userFeed.push(new Integer(tweetId));
    newsFeed.put(userId, userFeed);
    postToAllFollowers(userId, tweetId);
}

private void postToAllFollowers(int userId, int tweetId){
    HashSet<Integer> set = followList.get(userId);

    for(Integer followerId : set){
        Stack<Integer> followerFeed = newsFeed.get(followerId);
        followerFeed.push(new Integer(tweetId));
        newsFeed.put(followerId, followerFeed);
    }
}

/** Retrieve the 10 most recent tweet ids in the user's news feed. Each item in the news feed must be posted by users who the user followed or by the user herself. Tweets must be ordered from most recent to least recent. */
public List<Integer> getNewsFeed(int userId) {
    if(!newsFeed.containsKey(userId)){
        return null;
    }
    Stack<Integer> stack = newsFeed.get(userId);
    return stackToListIn10(stack);
}

private List<Integer> stackToListIn10(Stack<Integer> stack){
    List<Integer> timeLine = new ArrayList<Integer>();
    while((timeLine.size() != 10) || (!stack.isEmpty())){
        timeLine.add(stack.pop());
    }
    return timeLine;
}

/** Follower follows a followee. If the operation is invalid, it should be a no-op. */
public void follow(int followerId, int followeeId) {
    HashSet<Integer> set = followList.get(followerId);
    set.add(followeeId);
    followList.put(followerId, set);
}

/** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
public void unfollow(int followerId, int followeeId) {
    HashSet<Integer> set = followList.get(followerId);
    set.remove(followeeId);
    followList.put(followerId, set);
}
}

/**
 * Your Twitter object will be instantiated and called as such:
 * Twitter obj = new Twitter();
 * obj.postTweet(userId,tweetId);
 * List<Integer> param_2 = obj.getNewsFeed(userId);
 * obj.follow(followerId,followeeId);
 * obj.unfollow(followerId,followeeId);
 */

LeetCode gives me the following error: Line 20: java.lang.NullPointerException

Any insight on this? I haven't seen this problem on StackOverflow, so I don't think it's a duplicate