There is no other info?
No line number and no file name?
Attempt to call a nil value means you try to call a function somewhere, maybe from your XML code, that does not exist at the time you call it.
Well, disable ALL addons and libraries.
Test if no error.
If so: Enable ONLY your addon and LibAddonMenu-2.0.
If the error happens again:
Check your last changed files and see what you have changed there.
Download and enable DebugLogViewer and LibDebugLogger. At the settings menu of LibDebugLogger enable the strack traceback and at the UI for DebugLogViewer to left dropdown: Change to "Current session" (which is only current /reloadui then).
Check if there are any error messages shown in DebugLogViewer, which you normally would not see as they happen before the chat is ready.