I can reproduce this.
Clear campaign, no ext ect... GM host locally, Player client connects and assigns themselves a PC. GM client drags to the CT and the Player client gets the '[ERROR] Handler error: [string "ct/scripts/clientct.lua"]:28: attempt to index field 'summary_targets' (a nil value)' error.
It only throws for PCs which are assigned and have a player connected, not for PCs without connected players.

