Kotlin-Android-Extension doesn't work across module?

I'm using Kotlint 1.3.50 and Android Studio 3.4

I have my code

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        my_txt.text = "ABC"
    }
}

Any my layout as below

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    <TextView
            android:id="@+id/my_txt"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello World!"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent"/>

</android.support.constraint.ConstraintLayout>

Everything compiles fine if they are in the same module.

However, if I move my layout to an android library, with gradle as below

apply plugin: 'com.android.library'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 28


    defaultConfig {
        minSdkVersion 21
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }

}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])

    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

I can still import kotlinx.android.synthetic.main.activity_main.* and Android Studio doesn't complaint. But, when I compile, it complaints

Unresolved reference: activity_main 
Unresolved reference: my_txt    

Looks like the kotlin-extension can't cross use layout from another library?

If I change to findViewById<TextView>(R.id.my_txt).text = "XYZ", it works fine.

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        findViewById<TextView>(R.id.my_txt).text = "XYZ"
    }
}