Fix kotlin nullable errors in ThemePicker

Fix kotlin nullable errors that were exposed by setting the retention
of android.annotation.NonNull and android.annotation.Nullable to
class retention.

Bug: 294110802
Test: builds
Change-Id: If13ac9b81cfe65730d5ce270e9409bc97cef75ec
Merged-In: If13ac9b81cfe65730d5ce270e9409bc97cef75ec
diff --git a/src/com/android/customization/picker/color/ui/binder/ColorSectionViewBinder.kt b/src/com/android/customization/picker/color/ui/binder/ColorSectionViewBinder.kt
index 05b0916..9e232d7 100644
--- a/src/com/android/customization/picker/color/ui/binder/ColorSectionViewBinder.kt
+++ b/src/com/android/customization/picker/color/ui/binder/ColorSectionViewBinder.kt
@@ -95,7 +95,7 @@
                 LayoutInflater.from(view.context)
                     .inflate(R.layout.color_option_no_background, view, false)
             item.payload?.let { ColorOptionIconBinder.bind(itemView as ViewGroup, item.payload) }
-            val optionSelectedView = itemView.findViewById<ImageView>(R.id.option_selected)
+            val optionSelectedView = itemView.requireViewById<ImageView>(R.id.option_selected)
 
             lifecycleOwner.lifecycleScope.launch {
                 lifecycleOwner.repeatOnLifecycle(Lifecycle.State.STARTED) {