Fixes https://github.com/flutter/flutter/issues/124247

| Native | Flutter before | Flutter after |
| --- | --- | --- |
| <img width="248" alt="Screenshot 2023-04-05 at 9 26 16 AM" src="https://user-images.githubusercontent.com/389558/230177116-154999e8-eef3-441d-9fe9-7063839a6b99.png"> | <img width="240" alt="Screenshot 2023-04-05 at 11 18 01 AM" src="https://user-images.githubusercontent.com/389558/230177125-1680e851-223e-4956-b5b6-1a24e11dc22a.png"> | <img width="226" alt="Screenshot 2023-04-05 at 11 17 36 AM" src="https://user-images.githubusercontent.com/389558/230177123-bde82134-67e1-4ce2-8eec-719eeb779bf4.png"> |

Also, it's now possible for anyone to create disabled buttons like this by setting ContextMenuButtonItem.onPressed to `null`.