State of my Widget not changing in flutter when updating value - flutter

Question

What I am aiming for is, when a user long presses on a particular date, they should be able to enter their weight value, and upon submitting, the modal bottom sheet should close, and the updated weight should be visible as soon as the modal sheet is out of view. But It does not happen.
Instead I have to go to other date and come back to see the changes. Please help me. I am new to flutter.
Here is the demo of the problem :
problem i am facing
Here is the code:
void _modalBottomSheetMenu(DateTime dt) {
showModalBottomSheet(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(10.0),
),
context: context,
isScrollControlled: true,
builder: (builder) {
return Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
Text("Add a weight"),
Container(
width: 200.0,
child: TextField(
controller: myController,
keyboardType: TextInputType.number,
textAlign: TextAlign.center,
autofocus: true,
maxLength: 4,
onSubmitted: (value) {
weightValue = double.parse(value);
print("Weight entered is $weightValue");
print("Date passed is $dt");
setState(() {
_events[dt] = [weightValue].toList();
Navigator.pop(context);
});
print(_events[dt][0]);
},
decoration: InputDecoration(
border: OutlineInputBorder(),
labelText: 'Weight',
hintText: 'Enter your weight'),
),
),
],
);
},
);
}
BuildEventList : This contains the weight to be printed
import 'package:flutter/material.dart';
class BuildEventList extends StatefulWidget {
String weight;
BuildEventList(this.weight);
#override
_BuildEventListState createState() => _BuildEventListState();
}
class _BuildEventListState extends State<BuildEventList> {
#override
void setState(fn) {
// TODO: implement setState
super.setState(fn);
}
#override
Widget build(BuildContext context) {
return Container(
decoration: BoxDecoration(
border: Border.all(width: 0.8),
borderRadius: BorderRadius.circular(12.0),
),
margin: const EdgeInsets.symmetric(horizontal: 8.0, vertical: 4.0),
child: Container(
width: double.infinity,
child: Center(child: Text("${widget.weight}")),
),
);
}
}
This is where it is called :
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text(widget.title), actions: [
IconButton(
icon: Icon(Icons.add),
color: Colors.white,
onPressed: () {},
)
]),
body: Column(
mainAxisSize: MainAxisSize.max,
children: <Widget>[
_buildTableCalendar(),
const SizedBox(height: 8.0),
//_buildButtons(),
const SizedBox(height: 8.0),
Expanded(
child: BuildEventList(_selectedEvents.length > 0
? _selectedEvents[0].toString()
: "No weight given!")),
],
),
);
}