Hi! A simple bug report; I believe it's caused due to an interaction with the Omnimatter mod. It happens when I click on the little rock to find the new asteroids, and it tries to find the 'stone' entity, which is overridden by the Omnimatter entity.
Error:

Error while running event portal-research::on_gui_click (ID 1)
Unknown sprite "entity/stone"
stack traceback:
portal-research/modules/gui.lua:202: in function 'siteMiniDetails'
portal-research/modules/gui.lua:268: in function 'buildSitesList'
portal-research/modules/gui.lua:553: in function <portal-research/modules/gui.lua:528>

I also found this error when attempting to deconstruct the observatories: Error while running event portal-research::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
portal-research/control.lua:554: in function 'beltsMoveItems'
portal-research/control.lua:354: in function <portal-research/control.lua:351>

There is now an omnimatter integration by nucleargen that ensures only omnimatter will be generated on asteroids:
https://mods.factorio.com/mods/nucleargen/portal-research-addon

The invalid LuaEntity problem should have been fixed in recent versions but please let me know if anything like that happens again!

Thanks