The method 'contains' was called on null in FormBuilderCheckboxList

[shariful2011]

I am getting the following exception with FormBuilderCheckboxList:
I/flutter ( 3712): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 3712): The following NoSuchMethodError was thrown building
I/flutter ( 3712): FormField-[LabeledGlobalKey<FormFieldState>#9b66a](dirty, state:
I/flutter ( 3712): FormFieldState#3cadd):
I/flutter ( 3712): The method 'contains' was called on null.
I/flutter ( 3712): Receiver: null
I/flutter ( 3712): Tried calling: contains(0)
I/flutter ( 3712):
I/flutter ( 3712): When the exception was thrown, this was the stack:
I/flutter ( 3712): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5)
I/flutter ( 3712): #1 _FormBuilderCheckboxListState._checkbox
package:flutter_form_builder/…/fields/form_builder_checkbox_list.dart:55
I/flutter ( 3712): #2 _FormBuilderCheckboxListState._leading
package:flutter_form_builder/…/fields/form_builder_checkbox_list.dart:71
I/flutter ( 3712): #3 _FormBuilderCheckboxListState.build.
package:flutter_form_builder/…/fields/form_builder_checkbox_list.dart:108
I/flutter ( 3712): #4 FormFieldState.build (package:flutter/src/widgets/form.dart)

What's the way to work around?

Following is my code:
class

import 'package:flutter/material.dart';
import 'package:flutter_form_builder/flutter_form_builder.dart';

class MyCheckbox extends StatelessWidget {
final String attribute;
final List options;
final String title;
final bool mandatory;
final ValueChanged onChanged;
const MyCheckbox(
{Key key,
this.attribute,
this.options,
this.mandatory,
this.title,
this.onChanged})
: super(key: key);

String _labelText() {
return mandatory
? title.substring(0, 1).toUpperCase() + title.substring(1) + '*'
: title.substring(0, 1).toUpperCase() + title.substring(1);
}

@OverRide
Widget build(BuildContext context) {
return FormBuilderCheckboxList(
attribute: attribute,
options: options,
initialValue: null,
decoration: InputDecoration(
labelText: _labelText(),
labelStyle: TextStyle(
color: Color(0xFF4574EC),
),
border: InputBorder.none,
),
leadingInput: true,
validators: mandatory
? [
FormBuilderValidators.required(
errorText: 'Please select $title',
)
]
: [],
onChanged: onChanged,
);
}
}

calling it

MyCheckbox(
attribute: 'treatment',
mandatory: false,
title: 'Treatment received',
options: [
FormBuilderFieldOption(value: 0, label: 'IV Fluids'),
FormBuilderFieldOption(value: 1, label: 'Antibiotics'),
FormBuilderFieldOption(value: 2, label: 'ORS'),
],
onChanged: (dynamic val) {
print('treatmentreceived=$val');
setState(() {
_treatment = val;
});
},
),

[danvick]

Change initialValue: null, to initialValue: [],

[danvick]

Or alternatively don't include initialValue at all because its not needed.