Get java.lang.NumberFormatException: For input string: "null" when trying to show data in message

I keep coming across this issue, I'm trying to show the date the message has been posted in a messaging application and I keep getting this error and the application keeps crashing.

String message = chatList.get(position).getMessage();
        String timeStamp = chatList.get(position).getTimestamp();

        Calendar cal = Calendar.getInstance(Locale.ENGLISH);
        cal.setTimeInMillis(Long.parseLong(timeStamp));
        String dateTime = DateFormat.format("dd/MM/yyyy hh:mm aa", cal).toString();

        holder.messageTv.setText(message);
        holder.timeTv.setText(dateTime);

This is the logcat error I receive:

2020-09-02 22:10:08.411 13445-13445/com.x00122898.meetupproject E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.x00122898.meetupproject, PID: 13445
    java.lang.NumberFormatException: For input string: "null"
        at java.lang.Long.parseLong(Long.java:594)
        at java.lang.Long.parseLong(Long.java:636)
        at com.x00122898.meetupproject.ChatActivity$1.onDataChange(ChatActivity.java:107)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@16.0.4:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@16.0.4:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@16.0.4:55)
        at android.os.Handler.handleCallback(Handler.java:888)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:213)
        at android.app.ActivityThread.main(ActivityThread.java:8178)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1101)
2020-09-02 22:10:08.422 13445-13445/com.x00122898.meetupproject I/Process: Sending signal. PID: 13445 SIG: 9

Any help would be greatly appreciated ~

The problem is in these two lines of code:

String timeStamp = chatList.get(position).getTimestamp();
cal.setTimeInMillis(Long.parseLong(timeStamp));

Your variable timeStamp contains the string/word "null", and when you try to parse it as Long, it throws the NumberFormatException.
You must catch the exception or verify if timeStamp contains a numeric value (see How to check if a String is numeric in Java).

Also, maybe the method getTimestamp() is not returning the right variable or the timestamp was never saved or send

It looks like the problem occurs at ChatActivity.java line 107 where the value is literally "null". See previous answers
numberformatexception-for-input-string-null