Getting error "Smart cast to 'EditText!' is impossible, because 'usernameET' is a mutable property that could have been changed by this time"

Question

I m new to kotlin, I m trying to create an already developed app in Kotlin for learning purpose but getting the error. Below is the code;

class LoginActivity : AppCompatActivity() {

    private var usernameET: EditText? = null
    private var passwordET: EditText? = null
    private var usernameTIL: TextInputLayout? = null
    private var passwordTIL: TextInputLayout? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.login_layout);

        usernameET = findViewById(R.id.et_username)
        passwordET = findViewById(R.id.et_password)
        usernameTIL = findViewById(R.id.til_username)
        passwordTIL = findViewById(R.id.til_password)

        usernameET.addTextChangedListener(MyTextWatcher(usernameET))
        passwordET.addTextChangedListener(MyTextWatcher(passwordET))

        btn_login.setOnClickListener { view ->

        }
    }

    fun isUsernameValid(username: String): Boolean {
        if (username.isEmpty()) {
            this.usernameTIL.error = getString(R.string.username_error_msg);
            requestFocus(usernameET);
            return false;
        } else {
            this.usernameTIL.isErrorEnabled = false;
        }
        return true;
    }

    private fun isPasswordValid(password: String): Boolean {
        if (password.isEmpty()) {
            passwordTIL.error = getString(R.string.password_error_msg)
            requestFocus(passwordET)
            return false
        } else {
            passwordTIL.setErrorEnabled(false)
        }
        return true
    }

    fun requestFocus(view: View) {
        if (view.requestFocus()) {
            window.setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_VISIBLE)
        }
    }

    inner class MyTextWatcher(view: View) : TextWatcher {

        private var mView: View = view

        override fun afterTextChanged(p0: Editable?) {
            when (mView.id) {
                R.id.et_username -> {
                    isUsernameValid(usernameET.text.toString().trim())
                }

                R.id.et_password -> {
                    isPasswordValid(passwordET.text.toString().trim());
                }
            }
        }

        override fun beforeTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {

        }

        override fun onTextChanged(p0: CharSequence?, p1: Int, p2: Int, p3: Int) {

        }
    }
}

getting error "Smart cast to 'EditText!' is impossible, because 'usernameET' is a mutable property that could have been changed by this time" at line

usernameET.addTextChangedListener(MyTextWatcher(usernameET) )
passwordET.addTextChangedListener(MyTextWatcher(passwordET) )