As reported by user1 in rP19354, when pressing the massbarter hotkey in observermode without having a player view selected, an error is thrown:
ERROR: JavaScript error: gui/session/menu.js line 792 TypeError: GetSimState(...).players[g_ViewedPlayer] is undefined updateBarterButtons@gui/session/menu.js:792:18 __eventhandler378 (release)@__internal(40) release:1:4
This must be the only case where this could occur, as the trade window can never be opened when not having selected a playerview (and the other function already having an early return or receiving the market owner).
With this fix, it wouldn't throw errors even if it were possible (see also #4476).