The error AccessViolationException: "Attempted to read or write protected memory". What could be the problem?

In essence she was asked to do a dropdown list of users (combobox). After the list is selected by some user, it is added to a specific file, then a field combobox'again freely to enter.
Implementation: In the field combobox'a start to enter the name, there is a drop down list with users whose name coincide with the entered. The matching is implemented through a request to the IPA server(use JIRA), then the resulting list is processed and stored in an array, then the list is displayed in the combobox.

Problem: sometimes the code works consistently, but sometimes, every 4-5 requests the error AccessViolationException "Attempted to read or write protected memory. This often indicates that other memory is corrupt."
I would like to resolve this issue. Before the error appeared more often, I thought that the server blocks the abuse, so the request was to send every 4 characters. Then the problem was forgotten until the program used in the normal operating mode.

public void dropDownList(ComboBox comboBox)
{
 comboBox.Text = comboBox.Text;
 string text = comboBox.Text;

 string url = "https://serverAddress/api/2/user/picker?maxResults=7&query=" + text;

 HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);

 request.CookieContainer = new CookieContainer();
 request.Method = "GET";
 request.ContentType = "application/json; charset=utf-8";
request.CookieContainer.Add(Session.cookies);
try
{
 using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
{
 Stream stream = response.GetResponseStream();
 JsonSerializer jsonSerializer = new JsonSerializer();

 using (StreamReader reader = new StreamReader(stream))
{
 using (JsonTextReader jsonTextReader = new JsonTextReader(reader))
{
 var obj = jsonSerializer.Deserialize(jsonTextReader);
 string jsonString = @JsonConvert.SerializeObject(obj);

 JObject jObject = JObject.Parse(jsonString);
 JArray array = (JArray)jObject["users"];
comboBox.Items.Clear();
 comboBox.SelectionStart = text.Length;
 int count = array.Count;
 userArray = new string[count];
 string temp = "";

 for (int i = 0; i < count; i++)
{
 temp = string.Format("{0}", array[i]["html"]);
 temp = temp.Replace("strong>", "").Replace("<", "").Replace("/", "");
 userArray[i] = temp;
 temp = "";
}

comboBox.Items.AddRange(userArray);
 Array.Clear(userArray, 0, count);
}
}


}
}
 catch 
{
 MessageBox.Show("something went wrong");
}
 }