Add testing assert

components/constellation/constellation.rs

@@ -1618,6 +1618,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         let result = match self.pipelines.get(&pipeline_id) {
             None => return self.compositor_proxy.send(ToCompositorMsg::ChangePageTitle(pipeline_id, None)),
             Some(pipeline) => {
+                assert!(!pipeline.is_pending);
                 if !pipeline.is_pending {
                     pipeline.script_chan.send(ConstellationControlMsg::GetTitle(pipeline_id))
                 } else {