Avoid crashing renders given rendering races

If two OutPortals are ever rendered at the same time, the second
will steal the content from the first. When the first unmounts,
it accidentally unmounts the content from the second, so when
the second unmounts, it crashes.

This will always happen if you have two renders simultaneously,
and it seems it can also happen if they're not simultaneous but
maybe React has only started unmounting one element when it
mounts another (is this possible? seems to be happening, but
very unclear).

I'll extend this to avoid the issue entirely shortly, but this
fixes the immediate crash.

Author: pimterry

src/index.tsx

@@ -101,12 +101,15 @@ export class OutPortal<C extends Component<any>> extends React.PureComponent<Out
     }
 
     componentWillUnmount() {
-        const { node } = this.props;
-        node.parentNode.replaceChild(
-            this.placeholderNode.current,
-            node
-        );
-        this.props.node.setPortalProps({});
+        const node = this.props.node as PortalNode<C>;
+
+        if (node.parentNode) {
+            node.parentNode.replaceChild(
+                this.placeholderNode.current!,
+                node
+            );
+            this.props.node.setPortalProps({});
+        }
     }
 
     render() {