Stream.periodic clock is updating other StreamBuilder - flutter

Question

This is my code:
class MobileHomePage extends StatefulWidget {
const MobileHomePage({Key? key}) : super(key: key);
#override
State<MobileHomePage> createState() => _MobileHomePageState();
}
class _MobileHomePageState extends State<MobileHomePage> {
#override
void setState(fn) {
if (mounted) {
super.setState(fn);
}
}
int? second;
#override
void initState() {
second = int.parse(DateFormat.s().format(DateTime.now()));
Timer.periodic(const Duration(seconds: 1), (timer) => getCurrentTime());
super.initState();
}
void getCurrentTime() {
setState(() {
second = int.parse(DateFormat.s().format(DateTime.now()));
});
}
User user = FirebaseAuth.instance.currentUser!;
final Stream<QuerySnapshot> _mainSubjectStream = FirebaseFirestore.instance
.collection('systems')
.orderBy('system_name', descending: false)
.snapshots();
#override
Widget build(BuildContext context) {
double width = MediaQuery.of(context).size.width;
double height = MediaQuery.of(context).size.height;
final GlobalKey<ScaffoldState> _key = GlobalKey();
return Scaffold(
key: _key,
body: SafeArea(
bottom: false,
child: Column(
mainAxisAlignment: MainAxisAlignment.start,
children: [
Expanded(
flex: 1,
child: Container(
color: Colors.transparent,
child: StreamBuilder<QuerySnapshot>(
stream: _mainSubjectStream,
builder: (context, mainSubjectSnapshot) {
if (mainSubjectSnapshot.hasError) {
return const Center(child: Text('Error));
}
if (mainSubjectSnapshot.connectionState ==
ConnectionState.waiting) {
return const Center(
child: CircularProgressIndicator(),
);
}
var length = mainSubjectSnapshot.data!.docs.length;
return ListView.builder(
scrollDirection: Axis.horizontal,
itemCount: length,
itemBuilder: (context, i) {
var output = mainSubjectSnapshot.data!.docs[i];
return Text(output['name']);
});
},
),
)),
const Spacer(flex: 3),
Expanded(
flex: 5,
child: DatePicker(
DateTime.now(),
initialSelectedDate: _date,
daysCount: 8,
locale: 'pt',
selectionColor: const Color.fromRGBO(67, 97, 238, 1),
selectedTextColor: Colors.white,
onDateChange: (date){
setState(() {
_date = date;
});
},
),
),
Expanded(
flex: 1,
child: Text(second.toString()
),
],
),
));
}
}
When I try to display the clock (as in second.toString()), the StreamBuilder is also updated, resulting in a new progress indicator each second. What is breaking my head is that I just copied the same strutted that I used in other code – which works fine. Did I do something wrong? Why is it influencing the other stream?
EDIT
Have just found out that the error is related to the key in the scaffold. When I deleted it, it worked fine. The thing is that I need I as I have a custom button to open the drawer. 1st question is how does it influences it. 2nd question is how to solve it?