On Retail, with the latest versions of MM and WBT installed, clicking on the MM window to move it causes WBT to throw an error.

2x WorldBossTimers/GUI.lua:395: attempt to index field 'window' (a nil value)
[string "@WorldBossTimers/GUI.lua"]:395: in function <WorldBossTimers/GUI.lua:394>

Locals:
this = Button {
0 =
}
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index field 'window' (a nil value)"
self =

{
released = true
Rebuild = defined @WorldBossTimers/GUI.lua:169
UpdateContent = defined @WorldBossTimers/GUI.lua:214
UpdateWidth = defined @WorldBossTimers/GUI.lua:139
CyclicLabelResetTrigger = defined @WorldBossTimers/GUI.lua:199
RemoveLabel = defined @WorldBossTimers/GUI.lua:162
Restart = defined @WorldBossTimers/GUI.lua:45
InitPosition = defined @WorldBossTimers/GUI.lua:275
Show = defined @WorldBossTimers/GUI.lua:75
UpdatePosition = defined @WorldBossTimers/GUI.lua:259
NewBasicWindow = defined @WorldBossTimers/GUI.lua:322
height = 106
ShouldHide = defined @WorldBossTimers/GUI.lua:65
SetupAceGUI = defined @WorldBossTimers/GUI.lua:306
btn_req = {
}
Update = defined @WorldBossTimers/GUI.lua:248
ShouldShow = defined @WorldBossTimers/GUI.lua:53
btn_container = {
}
SaveGuiPoint = defined @WorldBossTimers/GUI.lua:286
CreateNewLabel = defined @WorldBossTimers/GUI.lua:112
CreateLabels = defined @WorldBossTimers/GUI.lua:41
btn_share = {
}
RecordPositioning = defined @WorldBossTimers/GUI.lua:297
LabelWidth = defined @WorldBossTimers/GUI.lua:108
visible = false
width = 200
Init = defined @WorldBossTimers/GUI.lua:37
AceGUI = {
}
ResetPosition = defined @WorldBossTimers/GUI.lua:301
New = defined @WorldBossTimers/GUI.lua:345
LockOrUnlock = defined @WorldBossTimers/GUI.lua:101
UpdateHeight = defined @WorldBossTimers/GUI.lua:128
CleanUpWidgetsAndRelease = defined @WorldBossTimers/GUI.lua:310
FreshKillTrigger = defined @WorldBossTimers/GUI.lua:185
Hide = defined @WorldBossTimers/GUI.lua:84
btn_opts = {
}
GetLabelText = defined @WorldBossTimers/GUI.lua:153
UpdateGUIVisibility = defined @WorldBossTimers/GUI.lua:93
}
GUI = {
released = true
Rebuild = defined @WorldBossTimers/GUI.lua:169
UpdateContent = defined @WorldBossTimers/GUI.lua:214
UpdateWidth = defined @WorldBossTimers/GUI.lua:139
CyclicLabelResetTrigger = defined @WorldBossTimers/GUI.lua:199
RemoveLabel = defined @WorldBossTimers/GUI.lua:162
Restart = defined @WorldBossTimers/GUI.lua:45
InitPosition = defined @WorldBossTimers/GUI.lua:275
Show = defined @WorldBossTimers/GUI.lua:75
UpdatePosition = defined @WorldBossTimers/GUI.lua:259
NewBasicWindow = defined @WorldBossTimers/GUI.lua:322
height = 106
ShouldHide = defined @WorldBossTimers/GUI.lua:65
SetupAceGUI = defined @WorldBossTimers/GUI.lua:306
btn_req = {
}
Update = defined @WorldBossTimers/GUI.lua:248
ShouldShow = defined @WorldBossTimers/GUI.lua:53
btn_container = {
}
SaveGuiPoint = defined @WorldBossTimers/GUI.lua:286
CreateNewLabel = defined @WorldBossTimers/GUI.lua:112
CreateLabels = defined @Interf

Thanks for reporting.

I couldn't reproduce it, but the reported nil access code looked problematic. Hopefully it's fixed by 240b080, which is now in latest version (v1.9.8). If you still see an issue, please re-open.