Flutter: Trying to display a widget conditionally based on whether a bool is true or false

I want to display a widget based on whether a variable that is a bool is true or false. However I get the error: _TypeError (type 'Null' is not a subtype of type 'Widget').

import 'package:expense_tracker/Models/expense.dart';
import 'package:flutter/material.dart';

class ExpensesItem extends StatelessWidget {
  const ExpensesItem(this.isAdditionalDisplayed, this.expense, {super.key,});

  final Expense expense;
  final bool isAdditionalDisplayed; //HERE IS THE BOOL VARIABLE

  _displayAdditional() {
    if(isAdditionalDisplayed == true) {
      Column(
        children: [
          Row(
            children: [
              SizedBox(
                height: 100,
                width: 360,
                child: Text(expense.additional))],
                )
              ],
            );
    }
    if(isAdditionalDisplayed == false) {
      SizedBox(height: 0,);
    }
  } //HERE I ASK IF _DISPLAYADDITIONAL IS TRUE OR FALSE. IF ITS TRUE IT DISPLAYS THE WIDGET, IF IT'S FALSE IT DISPLAYS AN EMPTY SIZEDBOX.

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Padding(
        padding: const EdgeInsets.symmetric(
          horizontal: 20,
          vertical: 16,
        ),
        child: Column(
          children: [
            Text(expense.title),
            const SizedBox(height: 4),
            Column(
              children: [
                Row(
                  children: [
                    SizedBox(
                      height: 100,
                      width: 360,
                      child: Text(expense.additional))],
                )
              ],
            ),
            _displayAdditional(), //HERE THE WIDGET SHOULD BE DISPLAYED. THIS IS WHERE I GET THE ERROR
          ],
        ),
      ),
    );
  }
}

Tried adding to the 2nd if statement to check if _isAdditionalDisplayed is null. This gives the error that it can't be null and will always be false:
if(isAdditionalDisplayed == false || isAdditionalDisplayed == null) {
Error:
The operand can't be null, so the condition is always 'false'.
Try removing the condition, an enclosing condition, or the whole conditional statement.

Tried adding an exclimation mark when _displayAdditional is called:
_displayAdditional()!,
Error:
_TypeError (Null check operator used on a null value)

With both of those changes at once I get both errors. With one or the other I get one or the other errors.

This is very confusing. In one place it's saying isAdditionalDisplayed can't be null. In another it's saying it is null.

Looks like the issue is you are not returning the widgets from the function.

import 'package:expense_tracker/Models/expense.dart';
import 'package:flutter/material.dart';

class ExpensesItem extends StatelessWidget {
  const ExpensesItem(this.isAdditionalDisplayed, this.expense, {super.key,});

  final Expense expense;
  final bool isAdditionalDisplayed;

  Widget _displayAdditional() {
    return isAdditionalDisplayed ? Column(
        children: [
          Row(
            children: [
              SizedBox(
                height: 100,
                width: 360,
                child: Text(expense.additional))],
                )
              ],
            ) : SizedBox.shrink();
  }

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Padding(
        padding: const EdgeInsets.symmetric(
          horizontal: 20,
          vertical: 16,
        ),
        child: Column(
          children: [
            Text(expense.title),
            const SizedBox(height: 4),
            Column(
              children: [
                Row(
                  children: [
                    SizedBox(
                      height: 100,
                      width: 360,
                      child: Text(expense.additional))],
                )
              ],
            ),
            _displayAdditional(), 
          ],
        ),
      ),
    );
  }
}