firebase-chat | Chapter 1 : Firebase Chat Application with Ionic | Authentication library

I've been starting to learn android since past few months. I'm creating a very basic chat application that uses Firebase Authentication, Firebase realtime database and Firebase Storage. I have used Lapit Chat Application YouTube tutorial and its source code in GitHub as the reference.

My application has MainActivity as the launcher activity, where the Firebase Authentication checks whether authentication is successful or not. If not, user is navigated to StartActivity (where user can login or register a new account). MainActivity has two fragments, namely ChatsFragment and FriendsFragment, which can be slided to navigate to each other (ViewPager is used).

Problem: The app crashes at startup when launched for the first time. After showing 'Unfortunately ChaTeX has stopped' and pressing okay, the app then launches, and everything works fine.

I am currently testing this application in Android Marshmallow in two devices where I get the same error in both devices. But the application doesn't launch at all in Nougat devices and the app keeps crashing all the time. What I guess is that the cause of error in both the android versions is same.

Logcat shows java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference in the onCreateView method of ChatFragments.java as shown in the snippet below(The line where I get Error is commented):

ChatsFragment.java:

I am fairly new to Firebase as well as angular2. So I am trying to make a Firebase integrated angular2 Chat app through tutorials. I have followed this tutorial to make the chat app. The problem is that addition to the simple chat, I want to create a toast when someone writes or receives a new chat. But I can't seem to find the code where I can listen to the that event.

Any idea where do I find it?

angular component looks something like this