alerts/sagas: avoid reentrancy for dismiss

This fixes react complaining about calling setState from an invalid
context.

src/alerts/sagas.ts

@@ -35,7 +35,9 @@ function* handleShowAlert(action: ReturnType<typeof alertsShowAlert>): Generator
     try {
         const alertAction = yield* take(chan);
         // the dismiss actions will have called this already, but other actions don't
-        toaster.dismiss(key);
+        if (alertAction !== 'dismiss') {
+            toaster.dismiss(key);
+        }
 
         yield* put(alertsDidShowAlert(action.domain, action.specific, alertAction));
     } finally {