Flutter date and time picker text form field onTap() clears other text form fields content

I am working on To Do Flutter app and I have a form to add new task with (title – description – deadline datetime) in the deadline TextFormField I made it readOnly and I set the onTap() to show the date picker and then show the time picker and then format it together and put it into the TextFormField, but the problem is when I enter the title and description and then I click on the deadline TextFormField to choose date and time, the tap clears what I typed in the title and description TextFormFields
this is the code

class AddTaskFormWidget extends StatelessWidget {
  AddTaskFormWidget({Key? key}) : super(key: key);

  static final GlobalKey<FormState> _formKey = GlobalKey<FormState>();

  final TextEditingController titleController = TextEditingController();
  final TextEditingController descriptionController = TextEditingController();
  final TextEditingController deadlineController = TextEditingController();
  
  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: [
          TextFormField(
            controller: titleController,
            decoration: const InputDecoration(
              label: Text('Task Title'),
            ),
            validator: (val) {
              if (!val!.isValidTitle) {
                return 'Please enter valid title';
              }

              return null;
            },
          ),
          TextFormField(
            controller: descriptionController,
            decoration: const InputDecoration(
              label: Text('Task Description'),
            ),
            validator: (val) {
              if (!val!.isValidDescription) {
                return 'Please enter valid description';
              }

              return null;
            },
          ),
          TextFormField(
            readOnly: true,
            onTap: () async {
              DateTime? deadlineDate = await showDatePicker(
                context: context,
                initialDate: DateTime.now(),
                firstDate: DateTime.now(),
                lastDate: DateTime(2100),
              );

              if (deadlineDate == null) return;

              TimeOfDay? deadlineTime = await showTimePicker(
                context: context,
                initialTime: TimeOfDay(
                  hour: 12,
                  minute: 0,
                ),
              );

              if (deadlineTime == null) return;

              final DateTime deadlineDateTime = DateTime(
                deadlineDate.year,
                deadlineDate.month,
                deadlineDate.day,
                deadlineTime.hour,
                deadlineTime.minute,
              );

              deadlineController.text = DateFormat('dd-MM-yyyy hh:mm').format(deadlineDateTime).toString();
            },
            controller: deadlineController,
            decoration: const InputDecoration(
              label: Text('Task Deadline'),
            ),
            validator: (val) {
              if (val!.isEmpty) {
                return 'Please enter deadline';
              }

              return null;
            },
          ),
          ElevatedButton(
            onPressed: () {
              if (_formKey.currentState!.validate()) {
                Navigator.pop(context);
              }
            },
            child: const Text('Submit'),
          ),
        ],
      ),
    );
  }
}