java - So I get the NullPointerException, how can I fix it?

So I'm just started to learn Java in school for like a month now, and I have this homework to make a simple Twitter-based social network. But I haven't learned database yet so we just making a simple database in the program.

Now the part where I get the error is that I instead of using switch, I used Reflection to call the methods in another class. And I get the NullPointerException error. I have read What is a NullPointerException, and how do I fix it? but I don't know how to fix it.

JavatterController.java

package jp.ac.kcg.java1;

public class JavatterController {
Javuser currentUser;
Database db;

public JavatterController(int index) {
    db = new Database();
    currentUser = db.getUser(index);
    System.out.println(currentUser.getUserIdWithAt() + " has logged in!");
}


public void tw(String tweet) {
    Tweet newTweet = new Tweet(tweet, currentUser.userId);
    db.insertTweet(newTweet);
    System.out.println(currentUser.getUserIdWithAt() + " just tweeted " + tweet);
}

Javatter.java

try {
                // load the AppTest at runtime
                Class<?> cls = Class.forName("jp.ac.kcg.java1.JavatterController");
                Object obj = cls.newInstance();
                // call the printItString method
                Method method = cls.getDeclaredMethod(input[0], paramString);
                method.invoke(obj, new String(input[1]));
            } catch(Exception ex) {
                ex.printStackTrace();
            }

And this is the error I got

@manofsteel > tw lol
@manofsteel > java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at jp.ac.kcg.java1.Javatter.main(Javatter.java:48)
Caused by: java.lang.NullPointerException
    at jp.ac.kcg.java1.JavatterController.tw(JavatterController.java:39)
    ... 5 more

So where did I do wrong and how can I fix it? Thanks in advance.