Unhandled Exception: setState() during building of sub class - flutter

Question

Im trying to pass data from a child widget to a parent widget using callback
so IM having this error
setState() or markNeedsBuild() called during build.
and I know its because of calling "callback" set msg before returning the stack(during building of child , superclass call setState :( )
widget.setmsg(msg);
return Stack(children: widget.model == posenet ? _renderKeypoints() : null);
SO how can I return renderKeypoints() widget and return msg to super? or is there a way to print msg immediately in the screen (I get the value of msg after having calculation in renderkeypoint ?
Subclass code :
typedef void IntCallback(String id);
// ignore: must_be_immutable
class BndBox extends StatefulWidget {
final List<dynamic> results;
final int previewH;
final int previewW;
final double screenH;
final double screenW;
final String model;
final IntCallback setmsg;
BndBox(this.results, this.previewH, this.previewW, this.screenH, this.screenW,
this.model, this.setmsg);
#override
_BndBoxState createState() => _BndBoxState();
}
class _BndBoxState extends State<BndBox> {
#override
Widget build(BuildContext context) {
String msg = "";
List<Widget> _renderKeypoints() {
var lists = <Widget>[];
var kx = [10.9];
kx.removeAt(0);
widget.results.forEach((re) {
var list = re["keypoints"].values.map<Widget>((k) {
var _x = k["x"];
var _y = k["y"];
var scaleW, scaleH, x, y;
kx.add(k["x"]);
if (widget.screenH / widget.screenW >
widget.previewH / widget.previewW) {
scaleW = widget.screenH / widget.previewH * widget.previewW;
scaleH = widget.screenH;
print("scaleW");
print(scaleW);
print("scaleH");
print(scaleH);
var difW = (scaleW - widget.screenW) / scaleW;
x = (_x - difW / 2) * scaleW;
y = _y * scaleH;
} else {
scaleH = widget.screenW / widget.previewW * widget.previewH;
scaleW = widget.screenW;
var difH = (scaleH - widget.screenH) / scaleH;
x = _x * scaleW;
y = (_y - difH / 2) * scaleH;
}
return Stack(children: [
Positioned(
left: x - 6,
top: y - 6,
width: 100,
height: 12,
child: Container(
child: Text(
"● ${k["part"]}",
style: TextStyle(
color: Color.fromRGBO(37, 213, 253, 1.0),
fontSize: 12.0,
),
),
),
),
]);
}).toList();
lists..addAll(list);
});
try {
if (((kx[3] - kx[5]).abs() <= 0.05) &&
((kx[3] - kx[11]).abs() <= 0.1)) {
msg = "RIGHT POSE";
} else if (((kx[3] - kx[5]) > 0.05 && (kx[3] - kx[5]) < 0.1) &&
(((kx[3] - kx[11]) > 0.1) && (kx[3] - kx[11]) < 0.2)) {
msg = "WRONG POSE";
} else {
msg = "UNDEFINED";
}
} catch (Exception) {
msg = "EXCEPTION";
}
return lists;
}
// widget.setmsg(msg); //error here
return Stack(
children: widget.model == posenet ? _renderKeypoints() : SizedBox(),
}
}

Answer 1

return Stack(children: widget.model == posenet ? _renderKeypoints() : SizedBox());

Answer 2

the solution does not need call back ,,
what I wanted has been solved
return Stack(children: <Widget>[
Stack(
children: widget.model == posenet ? _renderKeypoints() : null,
),
Text(msg)
]);