chore: Apply suggestion

Co-authored-by: Copilot <[email protected]>

Author: MartinZikmund

src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs

@@ -4848,7 +4848,10 @@ string GetMemberValue()
 			{
 				if (string.IsNullOrWhiteSpace(memberValue))
 				{
-					throw new InvalidOperationException($"The property value is invalid for {memberName} of type {propertyType?.Name}");
+					throw new InvalidOperationException(
+						$"The property value is invalid for {memberName}" +
+						(propertyType != null ? $" of type {propertyType.Name}" : "")
+					);
 				}
 
 				return memberValue!;