Bug 416432

STEPS TO REPRODUCE
1. if Dolphin is already running on your system, make sure "Undo" option is greyed out in "Edit" menu.
If not, close all instances of Dolphin and reopen it
2. to easily reproduce this issue, have a folder with some big files
3. copy or move your big files to another partition/device (Plasma shows a notification while Dolphin copies/moves the files)
4. wait Dolphin to copy/move some files and abort the file transfer before it to be completed by clicking on the stop button of Plasma notification
5. try to undo the partially completed file transfer via "Edit" menu > "Undo" or by pressing ctrl+z 

OBSERVED RESULT
"Undo" is greyed out in "Edit" menu and nothing happens when we press ctrl+z.

EXPECTED RESULT
We can undo the partially completed file transfer via "Undo" option of "Edit" menu or by pressing ctrl+z and we get the following results depending on which file operation was aborted:
if the files were copied, Dolphin shows a dialog asking if we want to delete from destination the files that were fully copied before the file transfer to be aborted;
if the files were moved, Dolphin moves the files that were fully moved to destination before the file transfer to be aborted back to the source location.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.17.90
KDE Frameworks Version: 5.66.0
Qt Version: 5.14.0