Cannot convert jobject element value to string

Question

public RecuperoStampaCodici(JObject responseSearch)

{

try

{

string code = String.Empty;

string licensecode = String.Empty;

string locationId = String.Empty;

//Scrivo i valori aggiornati di access_token e refresh_token

foreach (var valueJson in responseSearch)

{

if (valueJson.Key.Equals("Code"))

textBoxCode.Text = valueJson.Value.ToString();  //error NULL EXCEPTION

else if (valueJson.Key.Equals("License"))

textBoxLicenseCode.Text = valueJson.Value.ToString();

else if (valueJson.Key.Equals("LocationId"))

textBoxLocationId.Text = valueJson.Value.ToString();

}