cabana: update state after creating new signal (#27690)

old-commit-hash: 8bd4c0f510

tools/cabana/signaledit.cc

@@ -279,6 +279,7 @@ void SignalModel::handleSignalAdded(MessageId id, const cabana::Signal *sig) {
     beginInsertRows({}, i, i);
     insertItem(root.get(), i, sig);
     endInsertRows();
+    updateState(nullptr);
   }
 }