Error: automation error the object invoked has disconnected from its clients

Good day all,

I built a vba userform in my excel file and frequently receiving "automation error the object invoked has disconnected from its clients" error. Some times it works and some times it is not. I have comboboxes in the form, by choosing the comboboxes some times it does not show the range on itself and some times it just give me "Run Time" Error and close the excel.
it does confuse me because it happens some times not always.

I do appreciate in advance for your help.

Cheers.