LibHTML: Tear down the layout tree before changing the Frame's document

We don't want to deal with document().frame() being null inside layout
tree code, so this makes sure we tear it down before the frame has a
chance to get nulled out.

Libraries/LibHTML/HtmlView.cpp

@@ -52,11 +52,11 @@ void HtmlView::set_document(Document* document)
         };
     }
 
+    m_layout_root = nullptr;
+
     main_frame().set_document(document);
 
-    if (document == nullptr)
-        m_layout_root = nullptr;
-    else
+    if (document)
         m_layout_root = document->create_layout_tree(document->style_resolver(), nullptr);
 
 #ifdef HTML_DEBUG