[Catalyst] Fix NPE when open wifi calling screen

Bug: 379961379
Flag: com.android.settings.flags.catalyst_wifi_calling
Test: manual
Change-Id: I7c58ee786f5ca8ad76624d2c33d0375b3a0f1993

src/com/android/settings/wifi/calling/WifiCallingScreen.kt

@@ -37,6 +37,8 @@ class WifiCallingScreen : PreferenceScreenCreator {
 
     override fun fragmentClass() = WifiCallingSettingsForSub::class.java
 
+    override fun hasCompleteHierarchy() = false
+
     override fun getPreferenceHierarchy(context: Context) = preferenceHierarchy(this) {}
 
     companion object {