Firebase SignIn Using Email and Password Authentication in android studio and kotlin

class MainActivity : AppCompatActivity() {

    private var mauth = FirebaseAuth.getInstance()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(toolbar)
        requestedOrientation = ActivityInfo.SCREEN_ORIENTATION_PORTRAIT        val email = findViewById<View>(R.id.editT1) as EditText
        val password = findViewById<View>(R.id.editText4) as EditText
        val btn = findViewById<View>(R.id.button2) as Button
        val button = findViewById<View>(R.id.button4) as Button

        email.setOnFocusChangeListener { view, b ->            if (email.text.toString().contains("@") && 

                email.text.toString().contains(".com")) {
                toast("Valid Email Address")
                btn.isEnabled = true                button.isEnabled=true            }
            else {
                toast("Invalid Email Address")

                btn.isEnabled = false                button.isEnabled=false            }
        }

       btn.setOnClickListener {           toast("Logging In")
           mauth!!.signInWithEmailAndPassword(email.text.toString().trim(), 

                  password.text.toString().trim())
               .addOnCompleteListener(this){ task->                   if (task.isSuccessful){
                       toast("Login successful")
                       
                   }
                   else {
                       toast("Invalid Credentials Or User Not Registered")
                   }
               }       }