Flutter Validator String Requires Null Aware But Compiler Warns Not Needed

Question

I have a TextFormField:

TextFormField(
  textAlign:
      TextAlign.center,
  autovalidateMode:
      AutovalidateMode
          .onUserInteraction,
  onChanged: (value) {},
  controller:
      firstNameTextInputController,
  validator: (input) =>                         //////////////////////////// HERE
      (input!.length <
                  51 &&
              input!.length >
                  1)
          ? null
          : "Invalid First Name",                                    ////// End
  style: TextStyle(
    color: HexColor
        .fromHex(
            "#000000"),
  ),
  decoration:
      InputDecoration(
    border: InputBorder
        .none,
    filled: true,
    hintStyle: Theme.of(
            context)
        .textTheme
        .subtitle1!
        .merge(TextStyle(
            color: HexColor
                .fromHex(
                    "#707070"))),
    hintText:
        "*Enter First Name",
    fillColor: HexColor
        .fromHex(
            "#ffffff"),
    focusedBorder:
        OutlineInputBorder(
      borderSide: BorderSide(
          color: HexColor
              .fromHex(
                  "#707070"),
          width: 5),
      borderRadius:
          BorderRadius
              .circular(
                  5),
    ),
  ),
))),

This gives a warning:

warning: The '!' will have no effect because the receiver can't be null. (unnecessary_non_null_assertion at [athelite] lib\Pages\PlayerEditPageDefaultState.dart:427)

And so I remove the exclamation mark and then it turns into an error:

error: The property 'length' can't be unconditionally accessed because the receiver can be 'null'. (unchecked_use_of_nullable_value at [athelite] lib\Pages\PlayerEditPageDefaultState.dart:425)

There's just no pleasing the compiler! What is the correct way to do this with Flutter 2.0?

Answer 1

The first warning is listed in this documentation

The analyzer produces this diagnostic when the operand of the ! operator can’t be null.

This is because you are using the operator ! in 2 sides of the same && operator:

...
(input!.length < 51 && input!.length > 1)
...

If the first condition is fulfill, the second condition will operate on a non-null value of operand input, which produce the above warning.

To shut it down, simply removing the ! on the right side:

(input!.length < 51 && input.length > 1)