Not sure what causes it.

3x ...\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI-15.lua:467: attempt to call method 'SwingEnd' (a nil value)
[string "@interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI-15.lua"]:467: in function <...\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:466>
[string "@interface\SharedXML\C_TimerAugment.lua"]:16: in function <Interface\SharedXML\C_TimerAugment.lua:14>

Locals:
(*temporary) = nil
(*temporary) =

{
PLAYER_ENTER_COMBAT = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:620
ADDON_LOADED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:212
callbacks = {
}
PLAYER_ENTERING_WORLD = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:223
UNIT_SPELLCAST_START = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:552
UNIT_SPELLCAST_CHANNEL_START = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:580
PLAYER_EQUIPMENT_CHANGED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:609
UNIT_SPELLCAST_INTERRUPTED_OR_FAILED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:441
PLAYER_LEAVE_COMBAT = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:631
PLAYER_TARGET_CHANGED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:265
UnregisterAllCallbacks = defined @interface\AddOns\Masque\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:164
SwingTimerInfo = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:188
COMBAT_LOG_EVENT_UNFILTERED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:310
target = {
}
UNIT_SPELLCAST_INTERRUPTED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:473
RegisterCallback = defined @interface\AddOns\Masque\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:80
player = {
}
UNIT_ATTACK_SPEED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:385
UnregisterCallback = defined @interface\AddOns\Masque\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:143
UNIT_SPELLCAST_CHANNEL_STOP = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:590
UNIT_SPELLCAST_SUCCEEDED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:480
UnitSwingTimerInfo = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:198
UNIT_SPELLCAST_FAILED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:476
getUnit = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:178
}
(*temporary) = "offhand"
(*temporary) = "attempt to call method 'SwingEnd' (a nil value)"
self = {
PLAYER_ENTER_COMBAT = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:620
ADDON_LOADED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:212
callbacks = {
}
PLAYER_ENTERING_WORLD = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:223
UNIT_SPELLCAST_START = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:552
UNIT_SPELLCAST_CHANNEL_START = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:580
PLAYER_EQUIPMENT_CHANGED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:609
UNIT_SPELLCAST_INTERRUPTED_OR_FAILED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:441
PLAYER_LEAVE_COMBAT = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:631
PLAYER_TARGET_CHANGED = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:265
UnregisterAllCallbacks = defined @interface\AddOns\Masque\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:164
SwingTimerInfo = defined @interface\AddOns\LibClassicSwingTimerAPI\LibClassicSwingTimerAPI.lua:188
COMBAT_LOG_EVENT_UNFILTERED = defined @i

Fixed in release 2.0.1 https://github.com/Ralgathor/LibClassicSwingTimerAPI/releases/tag/v2.0.1.

Thanks for the report !