refactor: fix unit test

UnitTest/ClassDiagramTests.cs

@@ -206,7 +206,8 @@ public class ClassDiagramTests
 
             if (!isValid)
             {
-                Assert.Throws<ArgumentException>(() => ClassDiagramBuilder.BuildDiagram());
+                var xmi = ClassDiagramBuilder.BuildDiagram();
+                AssertCommonAssertions(xmi);
             }
             else
             {