Flutter DropdownButtonFormField same value error

I have two DropdownButtonFormfield on my page. They have relation. The second is filling after select an item from first. I am facing an error on this situation: Select an item from first, select an item from second, change the first value. I am seeing this error after apply this steps.

There should be exactly one item with [DropdownButtonFormField]'s value: 1789. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
dropdown.dart:1
Failed assertion: line 933 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'

My code :

DropdownButtonFormField<int?>(
        value: userController
         .selectedReturnCity,
        items: List.generate(
                  userController.storeCityFilter?.length ?? 0,
                   (index) => DropdownMenuItem(
                                         value: userController.storeCityFilter![index]['id'],
                                         child: Text(userController
                                                      .storeCityFilter![index]['value']))),
        onChanged: (value) async {
                      userController.selectedReturnCity = value == 0 ? null : value;
                      //Get data for second dropdown, the function is clears the data used by the second dropdown
                      await userController.getStores(city: userController.selectedReturnCity,
                                                  type: "market");
                  },
           ),
           10.v(),
DropdownButtonFormField<int?>(
        value: userController.selectedReturnStore,
        items: List.generate(
                  userController.storeList?.length ?? 0,
                   (index) => DropdownMenuItem<int?>(
                                         value: userController.storeList?[index].id,
                                         child: Text(userController.storeList?[index].name ?? ''))),
onChanged: (value) {userController.selectedReturnStore = value;
},
)