我怎样才能让这个javascript函数一直工作 (未捕获的 TypeError：无法读取 null 的属性)

我试图创建两个数字输入，每个都有自己的自定义"+"和"-"按钮.然而，在我当前的代码中，按钮功能似乎只在某些时候起作用.在其他情况下，我得到一个Uncaught TypeError: Cannot read properties of null (reading 'value') at HTMLButtonElement.<anonymous>错误.

这是HTML:

<div class="col">
    <div class="row">
        <div class="input-group mx-auto w-50">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm minus" data-id="one"
                    type="button"><i class="fa fa-minus"></i></button>
            </div>
            <input type="number" id="one" 
                class="form-control form-control-sm text-center" value="0">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm plus" data-id="one" type="button"><i
                        class="fa fa-plus"></i></button>
            </div>
        </div>
    </div>

    <div class="row">
        <div class="input-group mx-auto w-50">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm minus" data-id="two"
                    type="button"><i class="fa fa-minus"></i></button>
            </div>
            <input type="number" id="two"
                class="form-control form-control-sm text-center" value="0">
            <div class="input-group-prepend">
                <button class="btn btn-dark btn-sm plus" data-id="two" type="button"><i
                        class="fa fa-plus"></i></button>
            </div>
        </div>
    </div>
</div>

这是Javascript:

<script>
document.addEventListener("DOMContentLoaded", () => {
    document.querySelectorAll('.minus').forEach(item => {
        item.addEventListener('click', event => {
            var input_id = event.target.getAttribute('data-id')
            console.log('Minus button clicked for input ' + input_id)
            var quantityInput = document.getElementById(input_id);
            var currentQuantity = parseInt(quantityInput.value);
            quantityInput.value = currentQuantity - 1
        })
    })
    document.querySelectorAll('.plus').forEach(item => {
        item.addEventListener('click', event => {

            var input_id = event.target.getAttribute('data-id')
            console.log('Plus button clicked for input ' + input_id)
            var quantityInput = document.getElementById(input_id);
            var currentQuantity = parseInt(quantityInput.value);
            quantityInput.value = currentQuantity + 1
        })
    })
})

以下是单击几个按钮后的输出示例: