I got shot down in Dragonbane Keep, frantically trying to hit cat form before I landed. I manged to hit a lot of buttons, ended up in bear form with this error, which threw again when I reloaded.

9x ConRO_Druid/druid.lua:581: attempt to compare nil with number [string "@ConRO_Druid/druid.lua"]:581: in function NextSpell'
[string "@ConRO/core.lua"]:2433: in function `?'
[string "@DataStore/libs/AceTimer-3.0-17/AceTimer-3.0.lua"]:55: in function <DataStore/libs/AceTimer-3.0/AceTimer-3.0.lua:50>

Locals:
_ =

{
AbilityPurge = defined @ConRO/buttons.lua:2763
IsPvP = defined @ConRO/helper.lua:50
Dragonriding = defined @ConRO/helper.lua:1133
DestroyPurgableOverlays = defined @ConRO/buttons.lua:1684
modules = {
}
CancelTimer = defined @DataStore/libs/AceTimer-3.0/AceTimer-3.0.lua:147
AbilityInterrupt = defined @ConRO/buttons.lua:2723
ItemEquipped = defined @ConRO/helper.lua:147
PurgeWindowFrame = defined @ConRO/buttons.lua:1047
InterruptGlow = defined @ConRO/buttons.lua:1847
ScheduleTimer = defined @DataStore/libs/AceTimer-3.0/AceTimer-3.0.lua:94
TalentChosen = defined @ConRO/helper.lua:69
CreateDefenseOverlay = defined @ConRO/buttons.lua:1254
Heroism = defined @ConRO/helper.lua:434
ClassRGB = {
}
SetEnabledState = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:420
InvokeNextDef = defined @ConRO/core.lua:2467
ClearAbilityRaidBuffsIndependent = defined @ConRO/buttons.lua:2692
CreateRaidBuffsOverlay = defined @ConRO/buttons.lua:1518
InterruptFramePool = {
}
UnitAura = defined @ConRO/helper.lua:365
UNIT_ENTERED_VEHICLE = defined @ConRO/core.lua:2253
EnergyList = {
}
RotationTimer = {
}
ACTIONBAR_PAGE_CHANGED = defined @ConRO/core.lua:2408
DefSpells = {
}
optionsFrame = Frame {
}
CreateCoolDownOverlay = defined @ConRO/buttons.lua:1188
MovementFramePool = {
}
Spells = {
}
GetName = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:275
IsSpellInRange = defined @ConRO/helper.lua:858
Dump = defined @ConRO/buttons.lua:2600
Colors = {
}
RaidBuff = defined @ConRO/helper.lua:524
ClearAbilityInterruptIndependent = defined @ConRO/buttons.lua:2668
ToggleDamage = defined @ConRO/buttons.lua:718
Disable = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:310
PLAYER_CONTROL_LOST = defined @ConRO/core.lua:2309
Print = defined @ConRO/core.lua:2017
BurstMode = defined @ConRO/helper.lua:81
LEARNED_SPELL_IN_TAB = defined @ConRO/core.lua:2408
HideTauntGlow = defined @ConRO/buttons.lua:1955
DefFetchDiabolic = defined @ConRO/buttons.lua:2510
CreateBurstButton = defined @ConRO/buttons.lua:483
Totem = defined @ConRO/helper.lua:1121
SlashUnlock = defined @ConRO/buttons.lua:626
SetDefaultModuleLibraries = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:363
rotationEnabled = true
ItemReady = defined @ConRO/helper.lua:945
EnableModule = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:328
IterateEmbeds = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:438
RaidBuffsFramePool = {
}
DefenseFramePool = {
}
BlockAoE = defined @ConRO/buttons.lua:759
Interrupt = defined @ConRO/helper.lua:1059
UpdateButtonGlow = defined @ConRO/buttons.lua:1786
ACTIONBAR_HIDEGRID = defined @ConRO/core.lua:2235
defaultModuleState = true
Warnings = <fun`

Similar but not quite the same, this time when just changing specs in town. Equipped some new gear in Guardian, swapped to Feral and updated that gear, then got this error going back to Guardian.

2x ConRO_Druid/druid.lua:581: attempt to compare nil with number [string "@ConRO_Druid/druid.lua"]:581: in function NextSpell'
[string "@ConRO/core.lua"]:2458: in function `?'
[string "@DataStore/libs/AceTimer-3.0-17/AceTimer-3.0.lua"]:55: in function <DataStore/libs/AceTimer-3.0/AceTimer-3.0.lua:50>

Locals:
_ =

{
AbilityPurge = defined @ConRO/buttons.lua:2763
IsPvP = defined @ConRO/helper.lua:51
Dragonriding = defined @ConRO/helper.lua:1158
DestroyPurgableOverlays = defined @ConRO/buttons.lua:1684
modules = {
}
CancelTimer = defined @DataStore/libs/AceTimer-3.0/AceTimer-3.0.lua:147
DefFetchBlizzard = defined @ConRO/buttons.lua:2238
ItemEquipped = defined @ConRO/helper.lua:148
PurgeWindowFrame = defined @ConRO/buttons.lua:1047
InterruptGlow = defined @ConRO/buttons.lua:1847
ScheduleTimer = defined @DataStore/libs/AceTimer-3.0/AceTimer-3.0.lua:94
TalentChosen = defined @ConRO/helper.lua:70
CreateDefenseOverlay = defined @ConRO/buttons.lua:1254
Heroism = defined @ConRO/helper.lua:459
ClassRGB = {
}
SetEnabledState = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:420
InvokeNextDef = defined @ConRO/core.lua:2492
ClearAbilityRaidBuffsIndependent = defined @ConRO/buttons.lua:2692
CreateRaidBuffsOverlay = defined @ConRO/buttons.lua:1518
InterruptFramePool = {
}
UnitAura = defined @ConRO/helper.lua:390
UnregisterAllEvents = defined @AllTheThings/lib/CallbackHandler-1.0/CallbackHandler-1.0.lua:174
EnergyList = {
}
RotationTimer = {
}
ACTIONBAR_PAGE_CHANGED = defined @ConRO/core.lua:2433
DefSpells = {
}
optionsFrame = Frame {
}
CreateCoolDownOverlay = defined @ConRO/buttons.lua:1188
MovementFramePool = {
}
Spells = {
}
GetName = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:275
IsSpellInRange = defined @ConRO/helper.lua:883
Dump = defined @ConRO/buttons.lua:2600
Colors = {
}
RaidBuff = defined @ConRO/helper.lua:549
ClearAbilityInterruptIndependent = defined @ConRO/buttons.lua:2668
ToggleDamage = defined @ConRO/buttons.lua:718
Disable = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:310
PLAYER_CONTROL_LOST = defined @ConRO/core.lua:2334
Print = defined @ConRO/core.lua:2017
BurstMode = defined @ConRO/helper.lua:82
LEARNED_SPELL_IN_TAB = defined @ConRO/core.lua:2433
HideTauntGlow = defined @ConRO/buttons.lua:1955
DefFetchDiabolic = defined @ConRO/buttons.lua:2510
CreateBurstButton = defined @ConRO/buttons.lua:483
Totem = defined @ConRO/helper.lua:1146
SlashUnlock = defined @ConRO/buttons.lua:626
SetDefaultModuleLibraries = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:363
rotationEnabled = true
ItemReady = defined @ConRO/helper.lua:970
EnableModule = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:328
IterateEmbeds = defined @DataStore/libs/AceAddon-3.0/AceAddon-3.0.lua:438
RaidBuffsFramePool = {
}
DefenseFramePool = {
}
BlockAoE = defined @ConRO/buttons.lua:759
Interrupt = defined @ConRO/helper.lua:1084
UpdateButtonGlow = defined @ConRO/buttons.lua:1786
ACTIONBAR_HIDEGRID = defined @ConRO/core.lua:2244`