Why does Flutter need to have a "child: null" line of code when text button is used in a children[] widget?

New to flutter. I'm currently working through a flutter course where I practice adding packages to play sounds by building a xylophone. When adding multiple buttons for each sound file, flutter was telling me "The parameter 'child' is required" and has me insert it at the end of the TextButton with a null property. The code worked fine without it on there but dart analysis kept giving me a warning. Sample code after inserting child listed below.

 children: [
                TextButton(
                  onPressed: () {
                    final player = AudioCache();
                    player.play('note1.wav');
                  },
                  style: ButtonStyle(
                    backgroundColor: MaterialStateProperty.all<Color>(Colors.red),
                  ), child: null,

I cannot seem to find why this was warning me but reading the documentation it looks like maybe because it was expecting text for text button? Can anyone explain or point to why it does this?

Thanks in advance.

EDIT

As user @Saddan says, the TextButton class includes box constraints, meaning it will still have a size even without a child.

The real reason why you need a child widget is a bit more boring: There are very few reasons why you would want to have a TextWidget without a child, and even if you did, as I mention later in my answer, a more common approach to this is to use Container, which can be understood as an empty widget, because of this, the child property is considered required on the text widget (with null-safety, you get a compile-time error and without it, you get a lint warning like in the question).

The reason why child: null is added automatically is that flutter doesn't know what you want as your TextButton's child, but I believe you are meant to replace null with whatever else.

ORIGINAL ANSWER

A TextButton should always have a child because if it doesn't it will not display anything, a text button can't size itself so it will always have the same size as its child, you need to put something on the child property or the text button will not know what to look like.

Now, you are passing null as the child, I will have to admit, I didn't think that was possible, I also don't think flutter likes this idea very much (If you activate null-safety, this code will throw a compile-time error). I think you should instead use Container, which is an empty-by-default widget as the child property:

TextButton(
  onPressed: () {
    final player = AudioCache();
    player.play('note1.wav');
  },
  style: ButtonStyle(
    backgroundColor: MaterialStateProperty.all<Color>(Colors.red),
  ),
  child: Container(),

You can also use the container with some color, some width or some height if you want:

Container(
  color: Colors.red,
  width: 20,
  height: 20,
);

Your assumption is quite right. Its expecting a Widget ,if specific its expecting a TextWidget which consider a label or name of your button. Probably you're using an old version of flutter framework so its accepting null value of child but as I'm using 2.5.3 it's not accepting null value and you must need to provide it.