Don't walk treescopes in updateActiveStyleSheets until the resolver exists.

sky/engine/core/dom/StyleEngine.h

Index: sky/engine/core/dom/StyleEngine.h
diff --git a/sky/engine/core/dom/StyleEngine.h b/sky/engine/core/dom/StyleEngine.h
index 307f7a75e89db4e3214e69f24aa7a49de16f235c..ed17f6fe89fbb300e9abb0912acf2f8fec4f639e 100644
--- a/sky/engine/core/dom/StyleEngine.h
+++ b/sky/engine/core/dom/StyleEngine.h
@@ -59,8 +59,6 @@ public:
     void addTreeScope(TreeScope&);
     void removeTreeScope(TreeScope&);
 
-    void updateActiveStyleSheets();
-
     // FIXME(sky): Remove this and ::first-line.
     bool usesFirstLineRules() const { return false; }
 
@@ -102,6 +100,7 @@ private:
 private:
     explicit StyleEngine(Document&);
 
+    void updateActiveStyleSheets();
     void createResolver();
 
     RawPtr<Document> m_document;