Digital Light & Color

If you want to cancel the command Clone Entire Branch (because you click it instead of Clone Side-branch), all the tree is cloned all the same only with the dummy checkered image. There is no way to really cancel the cloning process. You have to wait until it completes and then close the tree.

Now with the feature File Open -> Select File you can Clone Entire tree with any image and replace in any time later with the right one, so there is no need for a temporary dummy tree.
In my opinion it would be better to reserve the Cancel button for cancelling the unwanted cloning.

The way this currently works is that the entire branch is clone more or less instantly, and the File Open command is reset to remove its pathname(s). Then the entire branch is recalculated. Recalculating a File Open with no pathname requests a pathname. If you cancel at this point, the clone operation has already been completed and the recalc does not know it was initiated by a clone operation so there is no easy way to cancel the clone operation. Canceling a recalc in progress runs the risk of leaving the image tree in an inconsistent state.

There may be some other way to make this work, but it will take some thinking.