Fixes #5078. Handle JSON null root in SourcesManager.Load (#5079)

* Initial plan

* Handle JSON null in SourcesManager.Load and add regression tests

Agent-Logs-Url: https://github.com/gui-cs/Terminal.Gui/sessions/866693ea-a3e8-40ba-9863-5053e0fbbd31

Co-authored-by: tig <585482+tig@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tig <585482+tig@users.noreply.github.com>
Co-authored-by: Tig <tig@users.noreply.github.com>

Author: Copilot

Terminal.Gui/Configuration/SourcesManager.cs

@@ -158,8 +158,14 @@ internal bool Load (SettingsScope? settingsScope, Stream stream, string source,
             stream.Position = 0;
             Debug.Assert (json != null);
 #endif
-            var scope = JsonSerializer.Deserialize (stream, typeof (SettingsScope), ConfigurationManager.SerializerContext.Options) as SettingsScope;
-            settingsScope.UpdateFrom (scope!);
+            SettingsScope? scope = JsonSerializer.Deserialize (stream, typeof (SettingsScope), ConfigurationManager.SerializerContext.Options) as SettingsScope;
+
+            if (scope is null)
+            {
+                throw new JsonException ("Configuration JSON root must not be null.");
+            }
+
+            settingsScope.UpdateFrom (scope);
             ConfigurationManager.OnUpdated ();
 
             AddSource (location, source);

Tests/UnitTests.NonParallelizable/Configuration/SourcesManagerLoadNullJsonTests.cs

@@ -0,0 +1,71 @@
+using System.Text;
+using System.Text.Json;
+using static Terminal.Gui.Configuration.ConfigurationManager;
+
+namespace UnitTests.NonParallelizable.ConfigurationTests;
+
+public class SourcesManagerLoadNullJsonTests
+{
+    [Fact]
+    public void Load_Stream_WithJsonNull_AndThrowOnJsonErrorsFalse_ReturnsFalse_AndAddsJsonError ()
+    {
+        // Copilot
+        SourcesManager sourcesManager = new ();
+        SettingsScope settingsScope = new ();
+        string source = "Load_Stream_WithJsonNull_AndThrowOnJsonErrorsFalse_ReturnsFalse_AndAddsJsonError";
+        ConfigLocations location = ConfigLocations.Runtime;
+        Stream stream = new MemoryStream (Encoding.UTF8.GetBytes ("null"));
+
+        bool? originalThrowOnJsonErrors = ThrowOnJsonErrors;
+        string originalJsonErrors = _jsonErrors.ToString ();
+
+        try
+        {
+            ThrowOnJsonErrors = false;
+            _jsonErrors.Clear ();
+
+            bool result = sourcesManager.Load (settingsScope, stream, source, location);
+
+            Assert.False (result);
+            Assert.Empty (sourcesManager.Sources);
+            Assert.Contains ($"Error reading {source}:", _jsonErrors.ToString ());
+        }
+        finally
+        {
+            ThrowOnJsonErrors = originalThrowOnJsonErrors;
+            _jsonErrors.Clear ();
+            _jsonErrors.Append (originalJsonErrors);
+        }
+    }
+
+    [Fact]
+    public void Load_Stream_WithJsonNull_AndThrowOnJsonErrorsTrue_ThrowsJsonException ()
+    {
+        // Copilot
+        SourcesManager sourcesManager = new ();
+        SettingsScope settingsScope = new ();
+        string source = "Load_Stream_WithJsonNull_AndThrowOnJsonErrorsTrue_ThrowsJsonException";
+        ConfigLocations location = ConfigLocations.Runtime;
+        Stream stream = new MemoryStream (Encoding.UTF8.GetBytes ("null"));
+
+        bool? originalThrowOnJsonErrors = ThrowOnJsonErrors;
+        string originalJsonErrors = _jsonErrors.ToString ();
+
+        try
+        {
+            ThrowOnJsonErrors = true;
+            _jsonErrors.Clear ();
+
+            JsonException exception = Assert.Throws<JsonException> (() => sourcesManager.Load (settingsScope, stream, source, location));
+            Assert.Contains ("null", exception.Message, StringComparison.OrdinalIgnoreCase);
+            Assert.Empty (sourcesManager.Sources);
+            Assert.Equal (0, _jsonErrors.Length);
+        }
+        finally
+        {
+            ThrowOnJsonErrors = originalThrowOnJsonErrors;
+            _jsonErrors.Clear ();
+            _jsonErrors.Append (originalJsonErrors);
+        }
+    }
+}