How do I give a variable a default value. Flutter/Dart

Question

I have this dropdown menu that other dropdown menus are dependent on. How do I give "selectedBusLine1" a default value that is stored in "'M${widget.busline1}'" so that "selectedBusLine1" is already populated instead of waiting for the onChanged. and if the user wanted to they can still choose a option in the dropdown list that will trigger the onChanged and overwrite the default value. Right now the "selectedBusLine1" is waiting to be populated by the on changed. Thanks a lot!

ListTileTheme(
                      tileColor: Colors.white,
                      child: ListTile(
                        title: Container(
                          decoration: BoxDecoration(
                            color: Colors.blueGrey[300],
                            border: Border.all(color: Colors.blueGrey),
                            borderRadius: BorderRadius.circular(10),
                            boxShadow: [
                              BoxShadow(
                                color: Colors.grey,
                                blurRadius: 4,
                                offset: Offset(4, 8), // changes position of shadow
                              ),
                            ],
                          ),
                          child: Padding(
                            padding: const EdgeInsets.all(8.0),
                            child: DropdownButtonHideUnderline(
                              child: DropdownButton<String>(
                                value: valueChoose_f13 ?? 'M${widget.busline1}',
                                isExpanded: true,
                                onChanged: (newValue){
                                  valueChoose_f14 = null;
                                  valueChoose_f15 = null;
                                  selectedBusLine1 = newValue == 'M100007'
                                      ? businessLine2_01
                                      : newValue == 'M100005'
                                      ? businessLine2_02
                                      : newValue == 'M100015'
                                      ? businessLine2_03
                                      : newValue == 'M100023'
                                      ? businessLine2_04
                                      : newValue == 'M100012'
                                      ? businessLine2_05
                                      : newValue == 'M100020'
                                      ? businessLine2_06
                                      : newValue == 'M100000'
                                      ? businessLine2_07
                                      : businessLine2_08;
                                  setState(() {
                                    valueChoose_f13 = newValue;
                                  });
                                },
                                items: businessLine1.map((map) {
                                  return DropdownMenuItem<String>(
                                    value: map['value'],
                                    child: Text(map['name']),
                                  );
                                }).toList(),
                              ),
                            ),
                          ),
                        ),
                      ),
                    ),