Fix Preferences apply silently discarding changes when a page fails to instantiate

Guard obj before calling apply() in PreferencesDialog to prevent a TypeError on
undefined from corrupting the ok accumulator. Also null-safe parent binding in
PreferencesPage to avoid the root cause during instantiation.

Fixes: #33841

Author: manolo

src/preferences/qml/MuseScore/Preferences/PreferencesDialog.qml

@@ -182,7 +182,9 @@ StyledDialogView {
                 for (var i in pages) {
                     var page = pages[i]
                     var obj = root.prv.pagesObjects[page.id]
-                    ok &= obj.apply()
+                    if (obj) {
+                        ok &= obj.apply()
+                    }
                 }
 
                 if (ok) {

src/preferences/qml/MuseScore/Preferences/PreferencesPage.qml

@@ -27,7 +27,7 @@ import Muse.UiComponents
 
 Rectangle {
     id: root
-    height: parent.height
+    height: parent ? parent.height : 0
     color: ui.theme.backgroundSecondaryColor
 
     default property alias contentData: content.data