Trouble creating a text typing effect?

Question

C++ / C#

I create the effect of typing text. The bottom line is this: There is a set of random characters in a string, a string with the desired text and two variables SpeedChanging (the speed of changing the random text), SpeedToSet (the speed of setting a character from the required string). It is necessary that the random text is re-generated with the speedChanging speed, and the required text from NeedText is set from left to right with the SpeedToSet speed.
For example:
needText = "Desired text"
First pass:
string result = akfgmassnvkczxvxzklc
Second pass:
string result = N fasdkmfgkvmxzckvvcs
Third pass:
string result = Well vbxcl,b;vxcmbmxcvsa
Fourth pass:
string result =Need vbxcl ,b;vxcmbmxcvs
and so on.
Here is how I am trying to do it:

[SerializeField]
    private Text mainText;
    [SerializeField]
    private string needText;
    private const string chars = "QqWwEeRrTtYyUuIiOoPpAaSsDdFfGgHhJjKkLlZzXxCcVvBbNnMmЙйЦцУуКкЕеНнГгШшЩщЗзХхЪъФфЫыВвАаПпРрОоЛлДдЖжЭэЯяЧчМмИиТтЬьБбЮю,.?/|\';:]{}[123456789+-*ҐґЄЇїє";
    [SerializeField]
    private int lenght;
    [SerializeField]
    private float speedChanging;
    [SerializeField]
    private float speedToSet;

    private void Start()
    {
        //mainText.text = GenerateRandom(lenght);
        StartCoroutine(GenerateString());
    }

    private string GenerateRandom(int lenght)
    {
        Random random = new Random();
        string result = "";

        for (int i = 0; i < this.lenght; i++)
        {
            result += chars[Random.Range(0, chars.Length)];
        }


        return result;
    }

    IEnumerator GenerateString()
    {
        for (int i = 0; i < needText.Length; i++)
        {
            mainText.text += needText[i];//генерирую нужный символ
            for (int j = i; j < needText.Length; j++)
            {
                mainText.text = mainText.text.Insert(j, "");//удаляю все что не нужно
            }
            mainText.text += GenerateRandom(needText.Length-i);// вставляю рандомные символы за нужными символами
            yield return new WaitForSeconds(speedChanging);
        }
    }

But on startup I get an error -

ArgumentOutOfRangeException: Specified argument was out of the range of valid values.
Parameter name: startIndex

When you click on it, it does not point to the place in the code where it appears from.

Answer 1

ArgumentOutOfRangeException
startIndex is negative or greater than the length of this instance.

From here - https://docs.microsoft.com/en-us/dotnet/api/system...
To insert at position j (which on the first iteration of the outer loop will be from 0 to needText.Length) in the line maintext.Text there should already be a sufficient number of characters - i.e. needText.Length.
Moreover - inserting an empty string does not make sense. Delete via substring, for example.