1 more type

Author: salazarm

packages/react-server/src/ReactFizzServer.js

@@ -240,7 +240,7 @@ export function createRequest(
   responseState: ResponseState,
   rootFormatContext: FormatContext,
   progressiveChunkSize: void | number,
-  onError: void | ((error: mixed) => string),
+  onError: void | ((error: mixed) => ?string),
   onAllReady: void | (() => void),
   onShellReady: void | (() => void),
   onShellError: void | ((error: mixed) => void),