EditTextPreference из библиотеки поддержки v7 сбой с NullPointerException

Приложение успешно компилируется, но вылетает при нажатии любого EditTextPreference.

SettingsActivity расширяет AppCompatActivity(). Это SettingsFragment.

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


class SettingsFragment : PreferenceFragmentCompat() {
    var key: String? = null


    override fun onCreatePreferences(savedInstanceState: Bundle?, rootKey: String?) {
        key = rootKey
        setPreferencesFromResource(R.xml.preferences, rootKey)
    }


    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        val topAppBarSettingsTitle: TextView = (activity as AppCompatActivity).findViewById(R.id.top_app_bar_settings_title)
        topAppBarSettingsTitle.text = if (key == null) "Settings" else findPreference(key).title
    }
}

Это соответствующая часть /res/xml/preferences.xml.

...
<PreferenceCategory android:title = "Personal info">
    <PreferenceScreen
        android:key = "nickname_screen"
        android:title = "Nickname"
        android:summary = "Nickname">
        <EditTextPreference
            android:key = "nickname"
            android:title = "Nickname"
            android:dialogTitle = "Nickname"
            android:hint = "Nickname"
            android:persistent = "true" />
        ...
    </PreferenceScreen>
</PreferenceCategory>
...

Это трассировка стека.

FATAL EXCEPTION: main
Process: com.aleph.null.messaging, PID: 5167
              java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.widget.EditText.requestFocus()' on a null object reference
                  at android.support.v7.preference.EditTextPreferenceDialogFragmentCompat.onBindDialogView(EditTextPreferenceDialogFragmentCompat.java:66)
                  at android.support.v7.preference.PreferenceDialogFragmentCompat.onCreateDialog(PreferenceDialogFragmentCompat.java:149)
                  at android.support.v4.app.DialogFragment.onGetLayoutInflater(DialogFragment.java:330)
                  at android.support.v4.app.Fragment.performGetLayoutInflater(Fragment.java:1292)
                  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1460)
                  at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784)
                  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852)
                  at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:802)
                  at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2623)
                  at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2410)
                  at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2365)
                  at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2272)
                  at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:733)
                  at android.os.Handler.handleCallback(Handler.java:873)
                  at android.os.Handler.dispatchMessage(Handler.java:99)
                  at android.os.Looper.loop(Looper.java:193)
                  at android.app.ActivityThread.main(ActivityThread.java:6669)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)