I have just created one SPFx web part with react framework. I tried testing through SharePoint online workbench (https://demo.sharepoint.com/sites/test/_layouts/15/workbench.aspx).
I am getting an error. Chrome browser developer tool shows below error:
Failed to load resource: the server responded with a status of 404 (Not Found) https://localhost:4321/node_modules/react-dom/dist/react-dom.js https://localhost:4321/node_modules/react/dist/react.js
Based on above mentioned error, I noticed the "node_modules/react/" does not contains "dist" folder at all.