Fix Advanced Paste settings page crash issue (#45207)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
This pull request refactors the `AdvancedPasteAdditionalActions` class
to use private backing fields and custom property accessors for its
action properties. This change allows for better control over property
initialization and ensures that the properties always have valid,
non-null default values.

**Refactoring for property initialization and null safety:**

* Introduced private backing fields (`_imageToText`, `_pasteAsFile`,
`_transcode`) for the `ImageToText`, `PasteAsFile`, and `Transcode`
properties in `AdvancedPasteAdditionalActions`, replacing
auto-properties.
* Updated the property accessors for `ImageToText`, `PasteAsFile`, and
`Transcode` to use the new backing fields and ensure that a new default
instance is assigned if a null value is provided during initialization.
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## PR Checklist

- [x] Closes: #45189
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issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
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pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
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- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/settings-ui/Settings.UI.Library/AdvancedPasteAdditionalActions.cs

@@ -10,6 +10,10 @@ namespace Microsoft.PowerToys.Settings.UI.Library;
 
 public sealed class AdvancedPasteAdditionalActions
 {
+    private AdvancedPasteAdditionalAction _imageToText = new();
+    private AdvancedPastePasteAsFileAction _pasteAsFile = new();
+    private AdvancedPasteTranscodeAction _transcode = new();
+
     public static class PropertyNames
     {
         public const string ImageToText = "image-to-text";
@@ -18,13 +22,25 @@ public sealed class AdvancedPasteAdditionalActions
     }
 
     [JsonPropertyName(PropertyNames.ImageToText)]
-    public AdvancedPasteAdditionalAction ImageToText { get; init; } = new();
+    public AdvancedPasteAdditionalAction ImageToText
+    {
+        get => _imageToText;
+        init => _imageToText = value ?? new();
+    }
 
     [JsonPropertyName(PropertyNames.PasteAsFile)]
-    public AdvancedPastePasteAsFileAction PasteAsFile { get; init; } = new();
+    public AdvancedPastePasteAsFileAction PasteAsFile
+    {
+        get => _pasteAsFile;
+        init => _pasteAsFile = value ?? new();
+    }
 
     [JsonPropertyName(PropertyNames.Transcode)]
-    public AdvancedPasteTranscodeAction Transcode { get; init; } = new();
+    public AdvancedPasteTranscodeAction Transcode
+    {
+        get => _transcode;
+        init => _transcode = value ?? new();
+    }
 
     public IEnumerable<IAdvancedPasteAction> GetAllActions()
     {