How can I fill google map markers ? flutter, custom icon - flutter

Question

enter image description here
enter image description here
It looks like this. The center is supposed to be filled with white, but it's transparent.
How can I fill this marker?
I used this code. It's from this link.
I wanted to set the marker's color to black.
So I used this code.
`
import 'dart:math';
import 'dart:ui';
import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
class MarkerGenerator {
final _markerSize;
double _circleStrokeWidth = 0;
double _circleOffset = 0;
double _outlineCircleWidth = 0;
double _fillCircleWidth = 0;
double _iconSize = 0;
double _iconOffset = 0;
MarkerGenerator(this._markerSize) {
// calculate marker dimensions
_circleStrokeWidth = _markerSize / 10.0;
_circleOffset = _markerSize / 2;
_outlineCircleWidth = _circleOffset - (_circleStrokeWidth / 2);
_fillCircleWidth = _markerSize / 3;
final outlineCircleInnerWidth = _markerSize - (2 * _circleStrokeWidth);
_iconSize = sqrt(pow(outlineCircleInnerWidth, 2) / 2);
final rectDiagonal = sqrt(2 * pow(_markerSize, 2));
final circleDistanceToCorners =
(rectDiagonal - outlineCircleInnerWidth) / 2;
_iconOffset = sqrt(pow(circleDistanceToCorners, 2) / 2);
}
/// Creates a BitmapDescriptor from an IconData
Future<BitmapDescriptor> createBitmapDescriptorFromIconData(IconData iconData,
Color iconColor, Color circleColor, Color backgroundColor) async {
final pictureRecorder = PictureRecorder();
final canvas = Canvas(pictureRecorder);
// _paintCircleFill(canvas, backgroundColor);
// _paintCircleStroke(canvas, circleColor);
_paintIcon(canvas, iconColor, iconData);
final picture = pictureRecorder.endRecording();
final image =
await picture.toImage(_markerSize.round(), _markerSize.round());
final bytes = await image.toByteData(format: ImageByteFormat.png);
return BitmapDescriptor.fromBytes(bytes!.buffer.asUint8List());
}
/// Paints the icon background
void _paintCircleFill(Canvas canvas, Color color) {
final paint = Paint()
..style = PaintingStyle.fill
..color = color;
canvas.drawCircle(
Offset(_circleOffset, _circleOffset), _fillCircleWidth, paint);
}
/// Paints a circle around the icon
void _paintCircleStroke(Canvas canvas, Color color) {
final paint = Paint()
..style = PaintingStyle.stroke
..color = color
..strokeWidth = _circleStrokeWidth;
canvas.drawCircle(
Offset(_circleOffset, _circleOffset), _outlineCircleWidth, paint);
}
/// Paints the icon
void _paintIcon(Canvas canvas, Color color, IconData iconData) {
final textPainter = TextPainter(textDirection: TextDirection.ltr);
textPainter.text = TextSpan(
text: String.fromCharCode(iconData.codePoint),
style: TextStyle(
letterSpacing: 0.0,
fontSize: _markerSize,
fontFamily: iconData.fontFamily,
color: color,
));
textPainter.layout();
// textPainter.paint(canvas, Offset(_iconOffset, _iconOffset));
textPainter.paint(canvas, Offset.zero);
}
}
`
I would like to fill in the marker.