Cant add item to a list in c#, i know why, but cant fix it

Question

So im making a config system, i know the code is mesy as ****, i try to add a item to a list but it fails cause the list seems to be: 'null'

i somehow need to refrence the: configElements list but i couldn't find out how, i dug through some docs and old stackoverflow questions, if this question has been answerd before im very sorry but i couldn't find the question, also sorry for my questionable english, its not my main language(?)

Here is the code i got so far, i would thank you very much if you could explain the issue.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

namespace ConfigTest
{
    class Config
    {
        private string configFile;
        private List<string> configElements;

        public Config(string configPath = "unset")
        {
            this.configFile = configPath;
        }

        //Checks if the config file allready exists
        public string makeConfig()
        {
            if (File.Exists(configFile))
            {
                return "Config file allready exists: " + configFile;
            }
            File.Create(configFile);
            return "Made Config file!";
        }

        public string getString(string element)
        {
            string[] configContent = getConfigContentL();
            foreach(string line in configContent)
            {
                if (line.Contains(element))
                {
                    return line.Replace(element + 1, "");
                }
                return "Couldn't find the element: " + element;
            }
            return "Couldn't open config file!";
        }

        private void appendElementToList(string element)
        {
            if (!this.configElements.Contains(element))
            {
                this.configElements.Add(element);
            }
        }

        public void setString(string element, string content)
        {
            string[] configContent = getConfigContentL();
            appendElementToList(element);
            if (configElements.Contains(element))
            {
                foreach(string line in configContent)
                {
                    if (line.Contains(element))
                    {
                        string editElement = getString(element);
                        line.Replace(editElement, content);
                    }
                }
            }
            else
            {
                File.AppendAllText(configFile , element + " " + content);
            }
        }



        public string getConfig()
        {
            return configFile;
        }

        public string getConfigContentF()
        {
            return File.ReadAllText(configFile);
        }

        public string[] getConfigContentL()
        {
            return File.ReadAllLines(configFile);
        }

        public List<string> getElements()
        {
            return configElements;
        }
    }
}