old bug , that I that I have the patience to report
if you unset DISPLAY and run okular gives a core dump , I saw this bug many years ago but still unsolved and now I can reproduce very easily , I found that because sometimes I do commands under screen session which doesn't set DSIPLAY ... I expect and easy fix which is a message can't run okular becauase can't open an X DISPLAY ...
to reproduce run :
unset DISPLAY
okular
qt.qpa.xcb: could not connect to display
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.
Aborted (core dumped)
best regards