Prevent users from starting an approval sequence where there are an approver in Episerver approval sequences

PreventSelfApprovalInit.cs

using EPiServer.Framework;
using EPiServer.Framework.Initialization;
using EPiServer.ServiceLocation;
using EPiServer.Core;
using EPiServer.Security;
using EPiServer.Approvals;
using EPiServer.Approvals.ContentApprovals;
using Microsoft.AspNet.Identity;
using Microsoft.AspNet.Identity.EntityFramework;
using EPiServer.Cms.UI.AspNetIdentity;

namespace ApprovalSequences
{
    [InitializableModule]
    [ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
    public class PreventSelfApprovalInit : IInitializableModule
    {
        public void Initialize(InitializationEngine context)
        {
            var contentEvents = ServiceLocator.Current.GetInstance<IContentEvents>();
            contentEvents.RequestingApproval += ContentEvents_RequestingApproval;
        }

        private void ContentEvents_RequestingApproval(object sender, EPiServer.ContentEventArgs e)
        {
            if (isUserInApprovalSequence(e.ContentLink))
            {
                e.CancelAction = true;
                e.CancelReason = "You are not allowed to approve your own content";
            }
        }

        private bool isUserInApprovalSequence(ContentReference contentRef)
        {
            var userName = PrincipalInfo.CurrentPrincipal.Identity.Name;
            var userManager = new UserManager<IdentityUser>(new UserStore<IdentityUser>(new ApplicationDbContext<IdentityUser>()));
            var allUserRoles = userManager.GetRoles(userManager.FindByName(userName).Id);

            var approvalRepo = ServiceLocator.Current.GetInstance<IApprovalDefinitionRepository>();
            var approvalSequence = approvalRepo.GetAsync(contentRef);
            
            foreach (var step in approvalSequence.Result.Steps)
            {
                foreach (var reviewer in step.Reviewers)
                {
                    if (reviewer.ReviewerType == ApprovalDefinitionReviewerType.User)
                    {
                        if (reviewer.Name == userName)
                            return true;
                    }
                    if (reviewer.ReviewerType == ApprovalDefinitionReviewerType.Role)
                    {
                        if (allUserRoles.Contains(reviewer.Name))
                            return true;
                    }
                }
            }

            return false;
        }

        public void Uninitialize(InitializationEngine context) { }
    }
}