Add unit test and fix failing new test

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Knowncolors.cs

@@ -666,6 +666,8 @@ internal static KnownColor ColorStringToKnownColor(string colorString)
 #if !PBTCOMPILER
         internal static KnownColor ArgbStringToKnownColor(string argbString)
         {
+            ArgumentNullException.ThrowIfNull(argbString);
+
             ReadOnlySpan<char> argbSpan = argbString.AsSpan().Trim();
 
             // Use NumberStyles.AllowHexSpecifier instead of NumberStyles.HexNumber because NumberStyles.HexNumber

src/Microsoft.DotNet.Wpf/tests/UnitTests/PresentationCore.Tests/System/Windows/Media/KnownColorsTests.cs

@@ -0,0 +1,26 @@
+namespace System.Windows.Media;
+
+public class KnownColorsTests
+{
+    [Theory]
+    // Supported values.
+    [InlineData(KnownColor.AliceBlue, "#FFF0F8FF")]
+    [InlineData(KnownColor.AliceBlue, " #FFF0F8FF")]
+    [InlineData(KnownColor.AliceBlue, " #FFF0F8FF ")]
+    [InlineData(KnownColor.AliceBlue, "#FFF0F8FF ")]
+    // Unsupported values.
+    [InlineData(KnownColor.UnknownColor, "")]
+    [InlineData(KnownColor.UnknownColor, " ")]
+    [InlineData(KnownColor.UnknownColor, "#020B37EF")] // Random ARGB that is not a known color.
+    [InlineData(KnownColor.UnknownColor, "# FFF0F8FF")]
+    public void ArgbStringToKnownColor_ReturnsExpected(object expected, string? argbString)
+    {
+        Assert.Equal((KnownColor)expected, KnownColors.ArgbStringToKnownColor(argbString));
+    }
+
+    [Fact]
+    public void ArgbStringToKnownColor_NullValue_ThrowsArgumentNullException()
+    {
+        Assert.Throws<ArgumentNullException>(() => KnownColors.ArgbStringToKnownColor(argbString: null));
+    }
+}