It seems that in recent versions something has changed with the NPC dialogue UI.

Now when adding commands to buttons it gives warnings for supported URLs.

From my understanding of this, the URL feature is only in education edition and has its own button, so this shouldn't show up in the commands section.

There has been no documentation to indicate that URL support was given to non education edition MC or that it had been implemented in a different way so this looks like a bug in the UI.