Fix falling in dev mode (#12407)

FiberNode stateNode could be null

So I get TypeError:

```
  at performWorkOnRoot (/tmp/my-project/node_modules/react-dom/cjs/react-dom.development.js:11014:24) TypeError: Cannot read property '_warnedAboutRefsInRender' of null
          at findDOMNode (/tmp/my-project/node_modules/react-dom/cjs/react-dom.development.js:15264:55)
```

Author: Yeti-or

packages/react-dom/src/client/ReactDOM.js

@@ -1126,7 +1126,7 @@ const ReactDOM: Object = {
   ): null | Element | Text {
     if (__DEV__) {
       let owner = (ReactCurrentOwner.current: any);
-      if (owner !== null) {
+      if (owner !== null && owner.stateNode !== null) {
         const warnedAboutRefsInRender =
           owner.stateNode._warnedAboutRefsInRender;
         warning(