If items in the list is changed then it throw error

There should be exactly one item with [DropdownButtonFormField]'s value: ... 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:dropdown_button2/src/dropdown_button2.dart':
Failed assertion: line 1045 pos 11: 'items == null ||


the items list has unique value but the items list change and want to reset the value to empty values