onActivtyResult.kt

 override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        // Result returned from launching the Intent from GoogleSignInApi.getSignInIntent(...);
        if (requestCode == RC_SIGN_IN) {
            val result = Auth.GoogleSignInApi.getSignInResultFromIntent(data)
            if (result.isSuccess) {
                // Google Sign In was successful, authenticate with Firebase
                val account = result.signInAccount
                firebaseAuthWithGoogle(account!!)
            } else {
                // Google Sign In failed
                Log.e("MainActivity", "Google Sign In failed.")
            }
        }
}

private fun firebaseAuthWithGoogle(acct: GoogleSignInAccount) {
        Log.d("Login", "firebaseAuthWithGoogle:" + acct.id!!)
        val credential = GoogleAuthProvider.getCredential(acct.idToken, null)
        mAuth.signInWithCredential(credential)
                .addOnCompleteListener(this, OnCompleteListener<AuthResult> { task ->
                    Log.d("Login", "signInWithCredential:onComplete:" + task.isSuccessful)

                    // If sign in fails, display a message to the user. If sign in succeeds
                    // the auth state listener will be notified and logic to handle the
                    // signed in user can be handled in the listener.
                    if (!task.isSuccessful) {
                        Log.d("Login", "signInWithCredential", task.exception)
                        Toast.makeText(this@LoginActivity, "Authentication failed.",
                                Toast.LENGTH_SHORT).show()
                    } else {
                        setLoginUIVisible(task.result?.user?.photoUrl)
                        textView_username.text = task.result?.user?.displayName
//                        textView_username.text = task.result?.user?.photoUrl.toString()
                        Glide.with(this@LoginActivity)
                                .load(task.result?.user?.photoUrl)
                                .into(imageView_profilePic)
                        setLogoutUI()
                    }
                })
    }