Problem on FormBuilderCheckboxList onChanged

[shariful2011]

This is my code for FormBuilderCheckboxlist:
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,
);

my calling:
MyCheckbox(
attribute: 'treatment',
mandatory: false,
title: 'Treatment received',
options: ['IV Fluids', 'Antibiotics', 'ORS']
.map(
(treatment) => FormBuilderFieldOption(value: treatment),
)
.toList(growable: false),
onChanged: (dynamic val) {
print('treatmentreceived=$val');
setState(() {
_treatment = val;
});
},
)
my UI http://prntscr.com/ntasjk

Whenever I tried to tap on the checkbox I got following exception:
I/flutter ( 6016): ══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter ( 6016): The following UnsupportedError was thrown while handling a gesture:
I/flutter ( 6016): Unsupported operation: Cannot add to an unmodifiable list
I/flutter ( 6016):
I/flutter ( 6016): When the exception was thrown, this was the stack:
I/flutter ( 6016): #0 UnmodifiableListBase.add (dart:_internal/list.dart:114:5)
I/flutter ( 6016): #1 _FormBuilderCheckboxListState._checkbox.
Please help.

[shariful2011]

Sorry, it was my mistake. Instead of List, I set String which causes the problem.