I think the Done button should be greyed out (unusable) if you haven't typed anything into the command block at all. However:

• It's not greyed out when you first open a command block.
• But it is greyed out if you type just one character.
• It's not greyed out if you then press backspace to delete that character.
• But it is greyed out if you then press backspace again.

And it's also greyed out if you copy-paste in a command.