Operation is not valid due to the current state of the object, Update web.config file using feature in SharePoint

Operation is not valid due to the current state of the object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.InvalidOperationException: Operation is not valid due to the current state of the object.
Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidOperationException: Operation is not valid due to the current state of the object.]    System.Web.HttpValueCollection.ThrowIfMaxHttpCollectionKeysExceeded() +4197167    System.Web.HttpValueCollection.FillFromEncodedBytes(Byte[] bytes, Encoding encoding) +60    System.Web.HttpRequest.FillInFormCollection() +189 [HttpException (0x80004005): The URL-encoded form data is not valid.]    System.Web.HttpRequest.FillInFormCollection() +11195688    System.Web.HttpRequest.get_Form() +119    Microsoft.SharePoint.SPGlobal.GetParametersFromHttpRequest(SPSite site, Boolean bAuthenticated) +422    Microsoft.SharePoint.SPGlobal.CreateSPRequestAndSetIdentity(SPSite site, String name, Boolean bNotGlobalAdminCode, String strUrl, Boolean bNotAddToContext, Byte[] UserToken, String userName, Boolean bIgnoreTokenTimeout, Boolean bAsAnonymous) +28024985    Microsoft.SharePoint.SPWeb.InitializeSPRequest() +223    Microsoft.SharePoint.WebControls.SPControl.EnsureSPWebRequest(SPWeb web) +365    Microsoft.SharePoint.WebControls.SPControl.SPWebEnsureSPControl(HttpContext context) +520    Microsoft.SharePoint.ApplicationRuntime.SPRequestModule.GetContextWeb(HttpContext context) +27    Microsoft.SharePoint.ApplicationRuntime.SPRequestModule.PostResolveRequestCacheHandler(Object oSender, EventArgs ea) +918    System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +80    System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +171

Open VS -> Create New Solution for SharePoint -> Select Features -> right click ‘Add New Feature’(make sure feature scope is 'WebApplication') -> select created new feature -> right click ‘Add Event receiver’

using System;

using System.Runtime.InteropServices;

using System.Security.Permissions;

using Microsoft.SharePoint;

using Microsoft.SharePoint.Security;

using Microsoft.SharePoint.Administration;

using System.Collections.ObjectModel;

namespace MyNewFeature.Features.UpdateWebConfig

{

    [Guid("20e0b198-3468-42cd-bf97-1d0bbef479eb")]

    public class MyNewFeatureEventReceiver : SPFeatureReceiver

    {

        public override void FeatureActivated(SPFeatureReceiverProperties properties)

        {

            SPWebApplication webApplication = properties.Feature.Parent as SPWebApplication;

            SPSecurity.RunWithElevatedPrivileges(delegate()

            {

                SPWebConfigModification mod = new SPWebConfigModification();

                mod.Path = "configuration/appSettings";

                mod.Name = "add [@key='aspnet:MaxHttpCollectionKeys'] [@value='10000']";

                mod.Owner = "MaxHttpCollectionKeys_Owner";

                mod.Sequence = 0;

                mod.Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode;

                mod.Value = "<add key='aspnet:MaxHttpCollectionKeys' value='10000' />";

                webApplication.WebConfigModifications.Add(mod);

                webApplication.Farm.Services.GetValue<SPWebService>().ApplyWebConfigModifications();

                webApplication.Update();                //ConfigurationManager.AppSettings["aspnet:MaxHttpCollectionKeys"].ToString();

            });

        }

        public override void FeatureDeactivating(SPFeatureReceiverProperties properties)

        {

            SPWebApplication webApplication = properties.Feature.Parent as SPWebApplication;

            SPSecurity.RunWithElevatedPrivileges(delegate()

            {

                Collection<SPWebConfigModification> mods = webApplication.WebConfigModifications;

                int initialModificationsCount = mods.Count;

                for (int i = initialModificationsCount - 1; i >= 0; i--)

                {

                    if (mods[i].Owner == "MaxHttpCollectionKeys_Owner")

                    {

                        SPWebConfigModification modToRemove = mods[i];

                        mods.Remove(modToRemove);

                    }

                }

                if (initialModificationsCount > mods.Count)

                {

                    webApplication.Farm.Services.GetValue<SPWebService>().ApplyWebConfigModifications();

                    webApplication.Update();

                }

            });

        }

    }

}