Don't know if this gives you enough information.

Date: 2019-09-01 08:33:12
ID: 1
Error occured in: Global
Count: 1
Message: ...rface\AddOns\BetterVendorPrice\BetterVendorPrice.lua line 218:
nil GetMouseFocus()
Debug:
[C]: error()
...rface\AddOns\BetterVendorPrice\BetterVendorPrice.lua:218:
...rface\AddOns\BetterVendorPrice\BetterVendorPrice.lua:189
[C]: ?
[C]: OnTooltipSetItem()
Enchantrix\EnxMain.lua:232:
Enchantrix\EnxMain.lua:228
[C]: ?
...AddOns\Auc-Advanced\Libs\LibExtraTip\LibExtraTip.lua:408:
...AddOns\Auc-Advanced\Libs\LibExtraTip\LibExtraTip.lua:402
[C]: SetHyperlink()
...ce\AddOns\AutoBarClassic\AutoBarClassBasicButton.lua:77: TooltipShow()
...ce\AddOns\AutoBarClassic\AutoBarClassPopupButton.lua:62:
...ce\AddOns\AutoBarClassic\AutoBarClassPopupButton.lua:55
[C]: ?
..\FrameXML\SecureHandlers.lua:266:
..\FrameXML\SecureHandlers.lua:263
[C]: ?
..\FrameXML\SecureHandlers.lua:322:
..\FrameXML\SecureHandlers.lua:306
(tail call): ?
Locals:
(*temporary) = "nil GetMouseFocus()"

AddOns:
Swatter, v8.2.6377 (SwimmingSeadragon)
WowheadLooter, v11302
AdvancedInterfaceOptions, v1.3.2
ArkInventoryClassic, v10004
ArkInventoryClassicRules, v10000
ArkInventoryClassicSearch, v10000
AtlasLootClassic, vv1.1.8-beta
AucAdvanced, v8.2.6385 (SwimmingSeadragon)
AucFilterBasic, v8.2.6364 (SwimmingSeadragon)
AucScanData, v8.2.6365 (SwimmingSeadragon)
AucStatHistogram, v8.2.6366 (SwimmingSeadragon)
AucStatiLevel, v8.2.6370 (SwimmingSeadragon)
AucStatPurchased, v8.2.6367 (SwimmingSeadragon)
AucStatSimple, v8.2.6368 (SwimmingSeadragon)
AucStatStdDev, v8.2.6369 (SwimmingSeadragon)
AucUtilFixAH, v8.2.6371 (SwimmingSeadragon)
AutoBarClassic, v1.13.2.00
BeanCounter, v8.2.6381 (SwimmingSeadragon)
BetterVendorPrice, vv1.05.00-classic
DBMCore, v1.13.5
DBMDefaultSkin, v
DBMStatusBarTimers, v
DejaClassicStats, v1302r012
Details, v
DetailsTinyThreat, v
Dominos, v8.2.16
DominosCast, v
DominosEncounter, v
DominosProgress, v
DominosRoll, v
DugisGuideViewerZ, v1.012
Enchantrix, v8.2.6392 (SwimmingSeadragon)
InFlightLoad, v1.13.002
Informant, v8.2.6374 (SwimmingSeadragon)
LunaUnitFrames, v1.0
MonkeyBuddy, v
MonkeyLibrary, v
MonkeyQuest, v
MonkeyQuestLog, v
OmniCC, v8.1.1
Peggle, v2.2.3
Prat30, v3.2.31
Prat30Libraries, v
Questie, v4.0.13 BETA
RealMobHealth, v2.3
SexyMap, vv2-classic
Stubby, v8.2.6376 (SwimmingSeadragon)
TitanClassic, v1.0.5.11302
TitanClassicAmmo, v1.0.5.11302
TitanClassicBag, v1.0.5.11302
TitanClassicClock, v1.0.5.11302
TitanClassicGold, v1.0.5.11302
TitanClassicLocation, v1.0.5.11302
TitanClassicLootType, v1.0.5.11302
TitanClassicPerformance, v1.0.5.11302
TitanClassicRepair, v1.0.5.11302
TitanClassicVolume, v1.0.5.11302
TitanClassicXP, v1.0.5.11302
TomTom, vv11302-1.0.7
tullaRange, v8.2.2
WeakAuras, v2.14.6
WeaponSwingTimer, v4.0.2
WhatsTraining, v1.6
BlizRuntimeLib_enUS v1.13.2.11302
(ck=79e)

Yes it's when there is nothing under your mouse, which is somewhat unexpected when dealing with tooltips

Is it some other addon showing some tooltip not under the mouse ?

@laric can you try https://www.curseforge.com/wow/addons/better-vendor-price/files/2776137 and confirm it fixes it but also tell me before you upgrade how you triggered the error?

Thanks!