When okular restarts if it can't find a file it throws a message box and spawns an empty pane. The latter is a PITA. Just rebooted after a system update and now I've got 20 or so unwanted okulars I need to kill off because of temp files I had open. Prblem is I'm impateitn and I'll just pkill okular rather than sort through the empty panes closing them one-by-one and that means I end up killing off non-empty ones. Basically why open a non-existent file?
Basically, because it's a session management feature. The system restarts every single application that was running in the previous session. It is hard to predict what was removed meanwhile. You can try to switch off some Okular messages with the "Show hints and info messages" option. https://docs.kde.org/trunk5/en/kdegraphics/okular/configgeneral.html Just my 2 cents.
I understand where it's coming from but my point is an error message and a blank pane double what is required! I could understand why the first instance opens a blank pane but the rest? clutter with no purpose.