Fix namespace null issue.

Author: ChebanovDD

src/UnityUxmlGenerator.UnityPackage/Assets/Plugins/UnityUxmlGenerator/Runtime/UnityUxmlGenerator.dll

@@ -9,11 +9,11 @@ protected BaseCapture(ClassDeclarationSyntax @class)
     {
         Class = @class;
         ClassName = @class.Identifier.Text;
-        ClassNamespace = @class.GetParent<NamespaceDeclarationSyntax>()!.Name.ToString();
+        ClassNamespace = @class.GetParent<NamespaceDeclarationSyntax>()?.Name.ToString();
     }
 
     public string ClassName { get; }
-    public string ClassNamespace { get; }
+    public string? ClassNamespace { get; }
     public abstract string ClassTag { get; }
 
     public ClassDeclarationSyntax Class { get; }

src/UnityUxmlGenerator/Captures/BaseCapture.cs

@@ -58,7 +58,7 @@ public override void OnVisitSyntaxNode(SyntaxNode syntaxNode)
         if (_captures.TryGetValue(@class!.Identifier.Text, out var uxmlTraits) == false)
         {
             uxmlTraits = new UxmlTraitsCapture(@class, @class.BaseList!.Types.First().Type);
-            _captures.Add(@class.Identifier.Text, uxmlTraits);
+            _captures.Add(uxmlTraits.ClassName, uxmlTraits);
         }
 
         uxmlTraits.Properties.Add((property, GetAttributeArgumentValue(attribute)));