Handle LoadOrCreate deserialization of JSON null via existing recovery path

AppDataStorage.Test/AppDataTests.cs

@@ -366,6 +366,20 @@ public void TestLoadOrCreateHandlesCorruptFile()
 		Assert.AreEqual(string.Empty, appData.Data, "Data should be default if loaded from corrupt file.");
 	}
 
+	[TestMethod]
+	public void TestLoadOrCreateHandlesNullJsonFile()
+	{
+		AbsoluteFilePath filePath = TestAppData.Get().FilePath;
+		AppData.EnsureDirectoryExists(filePath);
+		AppData.FileSystem.File.WriteAllText(filePath, "null");
+
+		TestAppData appData = TestAppData.LoadOrCreate();
+
+		AssertAppDataNotNull(appData, "LoadOrCreate should recover if file deserializes to null.");
+		Assert.AreEqual(string.Empty, appData.Data, "Data should be default if loaded from a null json file.");
+		Assert.IsTrue(AppData.FileSystem.File.Exists(filePath), "Recovery should recreate a valid settings file.");
+	}
+
 	[TestMethod]
 	public async Task TestMultipleSavesOnlyWriteOnceWithinDebouncePeriod()
 	{

AppDataStorage/AppData.cs

@@ -408,10 +408,10 @@
 	/// </summary>
 #if NET9_0_OR_GREATER
 	[JsonIgnore]
 	public static Lock Lock { get; } = new();
 #else
 	[JsonIgnore]
 	public static object Lock { get; } = new();
 #endif
 
 	internal bool IsSaveQueued()
@@ -515,7 +515,8 @@
 
 			try
 			{
-				newAppData = JsonSerializer.Deserialize<T>(jsonString, AppData.JsonSerializerOptions)!;
+				newAppData = JsonSerializer.Deserialize<T>(jsonString, AppData.JsonSerializerOptions)
+					?? throw new JsonException("Deserialized settings file to null.");
 				newAppData.Subdirectory = subdirectory;
 				newAppData.FileNameOverride = fileName;
 				return newAppData;