I have problem using this example

[KimTaeHyeong17]

User ID and nickname is successfully inputed and I can see it at my sendbird Dashboard
but when I click send button, app shut down with this error log message

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.sendbird.androidchattutorial.ChatActivity$ChatAdapter.sendMessage(java.lang.String)' on a null object reference
at com.sendbird.androidchattutorial.ChatActivity$2.onClick(ChatActivity.java:78)

How can I fix this??