Button onClick attribute is none if activity written in Kotlin

It seems like the designer does not support Kotlin yet. Here are some solution:

XML (Not Recommended)

Add the following line to your Button tag. This is exactly what the designer will do.

android:onClick="sendMessage"

Old Fashion

No need to add anything.

val button = findViewById<Button>(R.id.Button)
button.setOnClickListener {

}

kotlin-android-extensions (Recommended)

Add apply plugin: "kotlin-android-extensions" to your build.gradle

// button is the Button id
button.setOnClickListener {

}

Once defined the sendMessage class as :

/** Called when the user taps the Send button  */
fun sendMessage(view: View) {
    setContentView(R.layout.activity_second)
    // Do something in response to button
}

And also defined a second activity as:

class SecondActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_second)
    }
}

I added the SendMessage to the OnClick function:

And then it worked.

Your code will like this:

button.setOnClickListener(){
            Toast.makeText(this@MainActivity, "Its toast!", Toast.LENGTH_SHORT).show();
        }

Here import will:

import kotlinx.android.synthetic.main. activity_main.*

Here "button" is the id of that Button in .xml file. Here the advantage is no need to create Button object in your java class.