Update

src/ThemeEditor/Converters/ColorViewModelToBrushConverter.cs

@@ -10,26 +10,29 @@ namespace ThemeEditor.Converters;
 
 public class ColorViewModelToBrushConverter : IMultiValueConverter
 {
-    public object Convert(IList<object> values, Type targetType, object parameter, CultureInfo culture)
+    public object Convert(IList<object?>? values, Type targetType, object? parameter, CultureInfo culture)
     {
         if (values != null && values.Count() == 4)
         {
             for (int i = 0; i < 4; i++)
             {
-                if (values[i].GetType() != typeof(byte))
+                if (values[i]?.GetType() != typeof(byte))
                 {
                     return AvaloniaProperty.UnsetValue;
                 }
             }
 
-            var color = Color.FromArgb(
-                (byte)values[0],
-                (byte)values[1],
-                (byte)values[2],
-                (byte)values[3]);
+            var a = (byte?)values[0];
+            var r = (byte?)values[1];
+            var g = (byte?)values[2];
+            var b = (byte?)values[3];
+            if (a is { } && r is { } && g is { } && b is { })
+            {
+                var color = Color.FromArgb(a.Value, r.Value, g.Value , b.Value);
 
-            return new SolidColorBrush(color);
+                return new SolidColorBrush(color);
+            }
         }
         return AvaloniaProperty.UnsetValue;
     }
-}
+}

src/ThemeEditor/Converters/ThicknessViewModelToThicknessConverter.cs

@@ -9,23 +9,27 @@ namespace ThemeEditor.Converters;
 
 public class ThicknessViewModelToThicknessConverter : IMultiValueConverter
 {
-    public object Convert(IList<object> values, Type targetType, object parameter, CultureInfo culture)
+    public object Convert(IList<object?>? values, Type targetType, object? parameter, CultureInfo culture)
     {
         if (values != null && values.Count() == 4)
         {
             for (int i = 0; i < 4; i++)
             {
-                if (values[i].GetType() != typeof(double))
+                if (values[i]?.GetType() != typeof(double))
                 {
                     return AvaloniaProperty.UnsetValue;
                 }
             }
-            return new Thickness(
-                (double)values[0],
-                (double)values[1],
-                (double)values[2],
-                (double)values[3]);
+            
+            var l = (double?)values[0];
+            var t = (double?)values[1];
+            var r = (double?)values[2];
+            var b = (double?)values[3];
+            if (l is { } && t is { } && r is { } && b is { })
+            {
+                return new Thickness(l.Value, t.Value, r.Value, b.Value);
+            }
         }
         return AvaloniaProperty.UnsetValue;
     }
-}
+}

src/ThemeEditor/Preview/TabControlPage.axaml.cs

@@ -47,7 +47,7 @@ public class TabControlPage : UserControl
     private IBitmap LoadBitmap(string uri)
     {
         var assets = AvaloniaLocator.Current.GetService<IAssetLoader>();
-        return new Bitmap(assets.Open(new Uri(uri)));
+        return new Bitmap(assets?.Open(new Uri(uri)));
     }
 
     private class PageViewModel : ReactiveObject
@@ -69,4 +69,4 @@ public class TabControlPage : UserControl
         public string? Text { get; set; }
         public bool IsEnabled { get; set; } = true;
     }
-}
+}

src/ThemeEditor/ViewModels/ThemeEditorViewModel.cs

@@ -636,7 +636,7 @@ public class ThemeEditorViewModel : ReactiveObject
 
     private Window? GetWindow()
     {
-        if (Application.Current.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
+        if (Application.Current?.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
         {
             return desktopLifetime.MainWindow;
         }