Although it is possible to deselect a ship movement ghost without changing its move order, this does not hold true for omnis. if you select an omni via its green move ghost, you can't deselect the ghost by hitting cancel, you have to either choose a new spot for it to go to or try to remember exactly where you originally told it to go if you want it to go to the same spot it was going earlier.
This is due to a seemingly little-known feature of Omnis, where clicking the green ghost after it's been placed once will make the Omni follow it in real time. It's just not often noticed because people usually pause to move things like that. But none of this changes the fact that the inability to leave this state without placing it somewhere is indeed a bug. A quick bypass would be to always click the Omni itself to change its move location. That should get people by until a proper fix is done.
Certainly having a space-deselect while in this state return the move ghost to the original target would not be unwelcome.