25727539bb
Currently, the framework allows setting the clipboard to `null`. [Platforms handle this case inconsistently](https://github.com/flutter/flutter/issues/121976#issuecomment-1464721716), and we will update the framework to disallow setting the clipboard's data to `null`. In the meantime, this change makes the Windows embedder resilient to unexpected clipboard messages. If your clipboard data's text is unexpected, you will now get an error instead of crashing your app. Part of https://github.com/flutter/flutter/issues/121976