Flutter textformfield has 2 lines at the bottom

Question

I am trying to use TextFormField to manage user input, but there are two lines at the bottom, I want to get rid of the first thin line which sits in the middle of the text, but unable to do so.

My code is like this:

           Padding(
            padding: const EdgeInsets.symmetric(horizontal: 50.0),
            child: Form(
              key: _formKey,
              child: ConstrainedBox(
                constraints: BoxConstraints.tight(const Size(250, 50)),
                child: TextFormField(
                  maxLines: 1,
                  maxLength: 5,
                  maxLengthEnforced: true,
                  initialValue: '',
                  keyboardType: TextInputType.text,
                  autofocus: false,
                  decoration: InputDecoration(
                    helperText: 'Enter ref code',
                  ),
                  validator: _codeValidator,
                  onSaved: (value) => _inputCode = value.trim(),
                  style: textTheme.headline6.copyWith(letterSpacing: 40),
                ),
              ),
            ),
          ),

After searching for some answer, I try to modify the InputDecoration like this:

                  decoration: InputDecoration(
                    border: InputBorder.none,
                    helperText: 'Enter ref code',
                  ),

But it will get rid of the second line, the first ugly line is still there.

flutter doctor:

[✓] Flutter (Channel stable, 1.22.1, on Mac OS X 10.15.6 19G2021, locale en-AU)
 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 12.0.1)
[✓] Android Studio (version 3.6)
[✓] VS Code (version 1.50.0)
[✓] Connected device (1 available)

• No issues found!

Another issue is, even with maxLength: 5, and maxLengthEnforced: true, I can still input more than 5 characters, it does not automatically stop.

flutter error is not related to flutter doctor and flutter doctor is only for installing flutter not for codes and errors and you are not using general TextFormField you are using a package right? — Yaya, Oct 13 '20 at 10:12
The code you provided working fine,there is no problem with it, i have checked with dartpad there is no problem occurs.also you pointed out you can enter 5 number even if maxlength it is also working.can you show more code@xmanreturn — Abhijith, Oct 13 '20 at 10:18