Error while running event castra::on_selected_entity_changed (ID 59)
castra/control.lua:901: attempt to index field 'castra' (a nil value)
stack traceback:
castra/control.lua:901: in function 'clear_rect_range_render'
castra/control.lua:914: in function <castra/control.lua:908>

Same problem. Right after closing the tech screen.

I fixed it myself in control.lua adding an if storage.castra statement to the function clear_rect_range_render:

local function clear_rect_range_render(player)
if storage.castra then
if storage.castra.rendering_range_players and storage.castra.rendering_range_players[player.index] then
storage.castra.rendering_range_players[player.index].destroy()
storage.castra.rendering_range_players[player.index] = nil
end
end
end

Never messed with the code script, you got an easy to read step by step?