ui: Don't show reload toast

Bug:168619549
Change-Id: I6e7b760493d2888664e35a7a4c49d114bf58a804
diff --git a/ui/src/frontend/topbar.ts b/ui/src/frontend/topbar.ts
index e8403c3..02d29c9 100644
--- a/ui/src/frontend/topbar.ts
+++ b/ui/src/frontend/topbar.ts
@@ -190,7 +190,12 @@
 
 class NewVersionNotification implements m.ClassComponent {
   view() {
-    if (!globals.frontendLocalState.newVersionAvailable) return;
+    const engine: EngineConfig = globals.state.engines['0'];
+    // Don't show the new version toast if a trace is loading (engine exists).
+    if (!globals.frontendLocalState.newVersionAvailable ||
+        engine !== undefined) {
+      return;
+    }
     return m(
         '.new-version-toast',
         'A new version of the UI is available!',