pls don't panic

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

components/script/dom/document/document.rs

@@ -710,6 +710,7 @@ impl Document {
                 self.root_removal_noted.set(false);
 
                 if let Some(dirty_root) = self.dirty_root.get() {
+                    if dirty_root.is_connected() {
                         // There was an existing dirty root so we mark its
                         // ancestors as dirty until the document element.
                         for ancestor in dirty_root
@@ -721,6 +722,7 @@ impl Document {
                             }
                         }
                     }
+                }
                 self.dirty_root.set(Some(&document_element));
                 return;
             },