Without telling us the error message how should we be able to tell you if we get the same?
Please provide us more information:
-What do you mean by "in the Form" -> A context menu?
-All other addons disabled? If not do so and try again. If the error is gone try to find out which other addon is causing it
-Which addon is "yours" that uses the library LibCustomMenu and where you use AddCustomMenuItem?
-What to do to get the error message?
-Error message with all details AS TEXT, no screesnhot
(before raising the error: press ESC -> AddOns -> enable checkbox "Complete error message details")
-Source code file where the error is raised (at best attach the complete addon's zip file here, which raises the error, as attachment)