fix: fix image binding to stretch initialization issue.

.github/workflows/test.yml

@@ -12,12 +12,17 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4.1.1
+      - name: Ursa Unit Test
+        run: dotnet test ./tests/Test.Ursa
       - name: Ursa Headless Test
         run: dotnet test ./tests/HeadlessTest.Ursa
+
   ubuntu:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v4.1.1
+      - name: Ursa Unit Test
+        run: dotnet test ./tests/Test.Ursa
       - name: Ursa Headless Test
-        run: dotnet test ./tests/HeadlessTest.Ursa
+        run: dotnet test ./tests/HeadlessTest.Ursa

src/Ursa/Controls/ImageViewer/ImageViewer.cs

@@ -163,8 +163,9 @@ private void OnSourceChanged(AvaloniaPropertyChangedEventArgs args)
 
     private void OnStretchChanged(AvaloniaPropertyChangedEventArgs args)
     {
+        if (_image is null) return;
         var stretch = args.GetNewValue<Stretch>();
-        Scale = GetScaleRatio(Width / _image!.Width, Height / _image!.Height, stretch);
+        Scale = GetScaleRatio(Width / _image.Width, Height / _image.Height, stretch);
         _sourceMinScale = _image is not null ? Math.Min(Width * MinScale / _image.Width, Height * MinScale / _image.Height) : MinScale;
     }
 

tests/Test.Ursa/ImageViewerTests/Stretch_Initial_Value/StretchValueTest.cs

@@ -0,0 +1,17 @@
+using Avalonia.Controls;
+using Avalonia.Media;
+using Ursa.Controls;
+
+namespace Test.Ursa.Controls.ImageViewerTests.Stretch_Initial_Value;
+
+public class StretchValueTest
+{
+    [Fact]
+    public void Stretch_Initial_Value_Does_Not_Crash()
+    {
+        ImageViewer iv = new ImageViewer()
+        {
+            Stretch = Stretch.UniformToFill
+        };
+    }
+}