Initializing dropdownbutton in correct place in flutter code

I am trying to use this dropdown menu that has a list populated from a QuerySnapshot. I think I have it all set up right but it is causing this error:

There should be exactly one item with [DropdownButton]'s value: . 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':

I think this error has to do with the initialization of the Dropdown.

This is where I set the initial state of the dropdown. I assign the value from a document if it exists:

setState(() {
          _selectedInspectorCompany =
              trxnSnapshot.data()?['inspectorCompanyId'] ?? "";
        });

This is the code for the dropdown.

Wrap(

      children: [
        const Text(
          'Inspector Company:    ',
        ),
        StreamBuilder<QuerySnapshot>(
          stream: _db.collection('inspectorCompany').snapshots(),
          builder: (BuildContext context,
              AsyncSnapshot<QuerySnapshot> snapshot) {
            List<DropdownMenuItem<String>> inspectorCompanyItems =
                [];
            if (snapshot.hasData) {
              final inspectorCompanyList = snapshot.data!.docs;
              for (var inspectorCompany in inspectorCompanyList) {
                inspectorCompanyItems.add(
                  DropdownMenuItem<String>(
                    value: inspectorCompany.id,
                    child: Text(
                      inspectorCompany['inspectorCompanyName'],
                    ),
                  ),
                );
              }
              return DropdownButton<String>(
                hint: const Text("Select Inspector"),
                value: _selectedInspectorCompany,
                onChanged: (inspectorCompanyValue) {
                  setState(() {
                    _selectedInspectorCompany =
                        inspectorCompanyValue;
                  });
                  ref
                      .read(trxnNotifierProvider.notifier)
                      .updateInspectorCompanyId(
                          _selectedInspectorCompany!);
                },
                items: inspectorCompanyItems,
              );
            } else {
              return const Center(
                child: CircularProgressIndicator(),
              );
            }
          },
        ),
      ],
    ),

I think I am doing this correct but why am I getting the error?
Thanks

UPDATE:
If I add an inspectorCompany id to the transaction document so that it matches one of the inspectorCompany ids in the list of inspector companies it works fine.

What am I doing wrong? I think it may have to do with how I am initializing the _selectedInspectorCompany variable in the first code snippet but I don’t know what I am doing wrong.

Thanks.