Calling NotifyChanged on a negotiation card without a deck causes game to crash

function Card:NotifyChanged()
    local clone = self:Clone()
    if self.engine then
        -- Lock in the current damage and modifier string for this card, so that the UI
        -- will have the correct values generated from this point in time.
        local min_persuasion, max_persuasion, details = self.engine:PreviewPersuasion( self )
        clone.CalculatePersuasion = function( self )
            return min_persuasion, max_persuasion, details
        end
    end

    self.deck.engine:BroadcastEvent( EVENT.CARD_CHANGED, self, clone )
    return clone
end

title.

as you can see, on the third to last line, the self.deck part was completely unnecessary, as you can directly get the engine from self. However, if this is called on a card that's not in a deck, it will crash the game, since you're indexing on a nil value.