Error code Cannot read properties of null (reading 'appendChild) [duplicate]

I'm trying to run my code for my To do list project but keep getting the error (cannot read appendChild).

 let addToDoButton = document.getElementById('addToDo'); let toDoContianer = document.getElementById('toDoContianer'); let inputField = document.getElementById('inputField'); addToDoButton.addEventListener('click', function() { var paragraph = document.createElement('p'); paragraph.classList.add('paragraph-styling'); paragraph.innerText = inputField.value; toDoContianer.appendChild(paragraph); inputField.value = ""; paragraph.addEventListener('click', function() { paragraph.style.textDecoration = 'line-through'; }); paragraph.addEventListener('dblClick', function() { toDoContianer.removeChild(paragraph); }); });