I know this would be a rare case but atleast better than not fixing it. Currently in the AddTask() we only check with
string.IsNullOrEmpty(_taskText.value)
Still some can accidentally add some whitespaces and add the task down the list.
So an easy solution for this would be
private void AddTask()
{
if (string.IsNullOrEmpty(_taskText.value) || string.IsNullOrWhiteSpace(_taskText.value)) return;
// ...Code...
}