Kotlin throws a NullPointerException Using Kotlin's Android Extensions

Question

I'm using Kotlin to make an android app, but I keep getting a NullPointerException when trying to access a layout elements using Kotlin's android extensions. The code is a little bit complicated, so I'll try to explain the problem with a simple example. Say I have a layout second_layout.xml with a button and a textView:

<LinearLayout ...>

    <TextView
        android:id="@+id/textView"
        ..../>
    <Button
        android:id="@+id/button"
        android:onClick="onButtonClicked"
        ... />

</LinearLayout>

In the main activity, I want to change the textView text when I click on the button:

import kotlinx.android.synthetic.main.second_layout.*

override fun onCreate(savedInstanceState: Bundle?) {
    ...
}

fun onButtonClicked(view: View) {
    textView.text = "TextView text has changed"
}

Now when I click the button, Kotlin throw a NullPointerException, apparently the textView property is null. I hope the above code is clear.

Does anyone have an idea what's going on? Did I do something wrong? Thanks.