More Tip than Code Sharing.
I recall some times ago someone talking about Pasting a Function (with code) was not really working.
Today (minutes ago), I started to add code to a MenuItem, and realized my error: not the window into where I need that MenuItem to be active.
So, I Cut the MenuItem, and Pasted it in the correct location: the Paste was rejected silently.
So, I clicked in another part of that Window: same.
When I choosed Layout as the Paste target, it was accepted.
Note the green selection color.
THAT IS NOT THE BEST IDEA, because the first location to click is Menu Handler in that case (instead of Layout), but now we know what to do (provided we remember that).
I forget if I used Cmd-V or Right-Click → Paste (if that matters, who know ?).