Ensure DOM node exists before calling `node.removeChild` & `node.addChild`

[kyle-villeneuve]

After patching a very similar issue https://github.com/michalochman/react-pixi-fiber/pull/286 it's gotten a bit further downstream but it's still attempting to manipulate missing DOM nodes. Thus I'm introducing another null check to prevent that. I still think React.Suspense being somewhat incompatible with react-reconciler is the root cause but I'm hoping this workaround is sufficient for our needs🤞

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[michalochman]

Thanks @kyle-villeneuve!

[michalochman]

Included in react-pixi-fiber@1.0.4.