Unchecked Material Checkboxes not visible

Currently, I have to deal with a Flutter project that uses the flutter/material.dart package to display checkboxes, like so:

// ...
Checkbox(
    value: item.areaShowOnMap,
    activeColor: theme.colorPrimary,
    onChanged: (bool value){ //
      item.areaShowOnMap = value;
      if (value)
        _setCircle(item);
      else
        _deleteCircle(item);
    }
),
// ...

However the result looks like this:

This happens throughout the entire application on every checkbox. I tried to set the checkColor to Colors.black, but it's still invisible when it's unchecked, both on Android and iOS.

My question is: What could be the cause of the problem? I guess the inactive color is transparent, but I don't know how I can control this.

Check whether in your sample you set the ThemeData.checkBoxThemeData

Simple code snippet to reproduce your issue.

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

/// This is the main application widget.
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'Flutter Code Sample';

     Color getColor(Set<MaterialState> states) {
      const Set<MaterialState> interactiveStates = <MaterialState>{
        MaterialState.pressed,
        MaterialState.hovered,
        MaterialState.focused,
      };
      if (states.any(interactiveStates.contains)) {
        return Colors.brown;
      }
      return Colors.transparent;
    }
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      theme: ThemeData(
       checkboxTheme: CheckboxThemeData(
         fillColor: MaterialStateProperty.resolveWith(getColor)
       ),
      ),
      home: MyStatefulWidget(),
    );
  }
}

/// This is the stateful widget that the main application instantiates.
class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({Key? key}) : super(key: key);

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

/// This is the private State class that goes with MyStatefulWidget.
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  
  bool isSelected = false;
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        centerTitle: true,
        title: Text('CheckBox')
      ),
      body: Center(
        child: Checkbox(
         value: isSelected,
         activeColor: Theme.of(context).primaryColor,
         onChanged:(bool? value){
           setState((){
             isSelected = !isSelected;
           });
         } 
        ), 
      ),
    );
  }
}

Check whether your returned transparent in CheckBoxThemeData.fillColor

Color getColor(Set<MaterialState> states) {
          const Set<MaterialState> interactiveStates = <MaterialState>{
            MaterialState.pressed,
            MaterialState.hovered,
            MaterialState.focused,
          };
          if (states.any(interactiveStates.contains)) {
            return Colors.brown;
          }
          /// Instead of this you can return the Color                   
          return Colors.transparent;
        }