This doesn't happen on a copied level 60 character or a brand new level 1. The Count field is just rapidly incrementing until the client tells me to Disable Addons.

Message: ...\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:175: attempt to index field '?' (a nil value)
Time: Tue Apr 20 10:07:35 2021
Count: 1522
Stack: ...\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:175: attempt to index field '?' (a nil value)
[string "@interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua"]:175: in function UpdateRangeCastSpeedModifier' [string "@Interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua"]:218: in function UpdateAutoShotTimer'
[string "@interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua"]:252: in function `OnUpdate'
[string "@interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Core.lua"]:581: in function <...ce\AddOns\WeaponSwingTimer\WeaponSwingTimer_Core.lua:578>

Locals: weapon_id = nil
(*temporary) =

{
config_frame = WeaponSwingTimerHunterConfigPanel {
}
InitializeVisuals = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:533
guid = 0
has_moved = false
FontSizeEditBoxOnEnter = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:665
LoadSettings = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:122
is_spell_shoot = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:49
OnStopAutorepeatSpell = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:278
OnCombatLogUnfiltered = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:283
last_failed_time = 549338.715000
range_speed = 3
RestoreDefaults = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:140
OnStartAutorepeatSpell = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:269
UpdateRangeCastSpeedModifier = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:170
OnFrameDragStart = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:511
UpdateVisualsOnSettingsChange = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:442
StartCastingSpell = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:104
OnUnitSpellCastFailedQuiet = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:373
UpdateVisualsOnUpdate = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:388
UpdateConfigPanelValues = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:575
auto_cast_time = 0.520000
OOCAlphaOnValChange = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:757
OnUnitSpellCastSucceeded = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:309
OneBarCheckBoxOnClick = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:644
CooldownColorPickerOnClick = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:680
frame = WeaponSwingTimerHunterAutoshotFrame {
}
MultiClipColorPickerOnClick = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:728
HeightEditBoxOnEnter = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:660
last_shot_time = 549338.715000
UpdateInfo = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:151
shot_timer = -0.012000
ShowTextCheckBoxOnClick = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:650
YOffsetEditBoxOnEnter = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:675
XOffsetEditBoxOnEnter = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:670
EnabledCheckBoxOnClick = defined @interface\AddOns\WeaponSwingTimer\WeaponSwingTimer_Hunter.lua:619
OnUpdate = defined @interface\AddOns\Weapo`

What version?

6.3.2

If you turn the add-on off, then equip the bow, and reenable, does the error go away

fixed as of v6.3.3