error:The settings property 'cmb' was not found

public string cmb
     {
         get
         {
             return ((string)(this["cmb"]));
         }
         set
         {
             this["cmb"] = value;
              
         }
     }

<configSections>
      <sectionGroup name="userSettings" type="System.Configuration.UserSettingsGroup, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" >
        <section name="test_TextBox.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" allowExeDefinition="MachineToLocalUser" requirePermission="false" />
      </sectionGroup>
    </configSections>
  <userSettings>
    <test_TextBox.Properties.Settings>
      <setting name="cmb" serializeAs="String">
        <value />
      </setting>
    </test_TextBox.Properties.Settings>
  </userSettings>

private void TXTvolt_KeyPress(object sender, KeyPressEventArgs e)
       {
           if (e.KeyChar==13)
           {
               if (!Properties.Settings.Default.cmb.Contains("," + TXTvolt.Text.TrimStart().TrimEnd().Trim() + ","))
               {
 
                   Properties.Settings.Default.cmb += TXTvolt.Text.TrimStart().TrimEnd().Trim() + ",";
                   Properties.Settings.Default.Save();
 
               }
               TXTvolt.Items.Add(TXTvolt.Text);
           }
       }