How make default selection in FormBuilderDropdown list in flutter?

I am having a multiple FormBuilder dropdown in my flutter app. all of them are dependent dropdown. i want to implement something like this:

1. the first of the main parent dropdown value should be auto fill and hence all other are filled or selected depending on the parents selection.
2. I can see the text filled in all the dropdown but as it is the dependent it is disabled for first dropdown to be clicked once.

How to fix this issue?

in the below screenshot we can see that the data is filled or been selected in every dropdown but second dropdown is still been disabled. it only gets enabled after clicking or selecting the value in first dropdown. enter image description here

Here is my code for first dropdown

                padding: const EdgeInsets.all(12.0),
                child: FormBuilderDropdown(
                  name: 'City',
                  decoration: InputDecoration(
                    labelText: 'City',
                  ),
                  allowClear: true,
                
                  hint: Text(widget.callLocationModel.CityName),
                  initialValue: citySelected,

                  validator: FormBuilderValidators.compose(
                      [FormBuilderValidators.required(context)]),
                  items: cityList
                      .map((citySelected) => DropdownMenuItem(
                          value: cityChoosen,
                          child:new Text('$citySelected'),
                          ))
                      .toList(),
                  onChanged: (String value){
                      setState(() {
                        citySelected= value;
//this fuction calls my second dropdown depending on first dropdown value selection
                        fetchSite(citySelected);
                      });
                  },
                ),
              ),