Auto merge of #22411 - jdm:fix-webdriver, r=SimonSapin

Make desktop webdriver WPT harness work

This change allows me to run `./mach test-wpt --product servodriver --no-pause-after-test tests/wpt/mozilla/tests/mozilla/DOMParser.html` successfully again.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22409
- [x] There are tests for these changes

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22411)
<!-- Reviewable:end -->

1 files changed, 6 insertions, 7 deletions

diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index afb0c16cde5..8195e4efc77 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -4034,17 +4034,16 @@ where
 
     /// Send the current frame tree to compositor
     fn send_frame_tree(&mut self, top_level_browsing_context_id: TopLevelBrowsingContextId) {
-        self.active_browser_id = Some(top_level_browsing_context_id);
-        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
-
         // Note that this function can panic, due to ipc-channel creation failure.
         // avoiding this panic would require a mechanism for dealing
         // with low-resource scenarios.
-        debug!(
-            "Sending frame tree for browsing context {}.",
-            browsing_context_id
-        );
+        let browsing_context_id = BrowsingContextId::from(top_level_browsing_context_id);
         if let Some(frame_tree) = self.browsing_context_to_sendable(browsing_context_id) {
+            debug!(
+                "Sending frame tree for browsing context {}.",
+                browsing_context_id
+            );
+            self.active_browser_id = Some(top_level_browsing_context_id);
             self.compositor_proxy
                 .send(ToCompositorMsg::SetFrameTree(frame_tree));
         }