Create Users and Roles in ASP.NET 6 & Entity Framework 7

Question

I need to create users and roles at the first startup of my web application. For that, I write my code into the Configure() function in Startup.cs.

When I run my application, I get this error:

An exception of type 'System.AggregateException' occurred in mscorlib.dll but was not handled in user code at the call of my first function "CreateUsers"

Code:

 public async void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, IServiceProvider serviceProvider, ApplicationDbContext context)
{
   ...
        await CreateUsers(context, serviceProvider);
        await CreateRoles(context, serviceProvider);
    }

    private async Task CreateUsers(ApplicationDbContext context, IServiceProvider serviceProvider)
    {
        List<ApplicationUser> users = new List<ApplicationUser>();
        users.Add(new ApplicationUser { UserName = "devUser", Email = "[email protected]", LastName = "dev", FirstName = "dev", PasswordHash = "devpassword" });
        users.Add(new ApplicationUser { UserName = "workUser", Email = "[email protected]", LastName = "work", FirstName = "work", PasswordHash = "workPassword" });

        foreach(var user in users)
        {
            var userExist = await _userManager.FindByEmailAsync(user.Email);
            if(userExist != null)
            {
                await _userManager.CreateAsync(user, user.PasswordHash);
                context.Users.Add(user);
                context.SaveChanges();
            }
        }
    }

    private async Task CreateRoles(ApplicationDbContext context, IServiceProvider serviceProvider)
    {
        var UserManager = serviceProvider.GetRequiredService<UserManager<ApplicationUser>>();
        var RoleManager = serviceProvider.GetRequiredService<RoleManager<IdentityRole>>();

        List<IdentityRole> roles = new List<IdentityRole>();
        roles.Add(new IdentityRole { Name = "Developer", NormalizedName = "DEVELOPER" });
        roles.Add(new IdentityRole { Name = "Work", NormalizedName = "WORK" });
        roles.Add(new IdentityRole { Name = "Admin", NormalizedName = "ADMIN" });
        roles.Add(new IdentityRole { Name = "Redacteur", NormalizedName = "REDACTEUR" });
        roles.Add(new IdentityRole { Name = "Correcteur", NormalizedName = "CORRECTEUR" });
        roles.Add(new IdentityRole { Name = "Membre", NormalizedName = "MEMBRE" });

        foreach(var role in roles)
        {
            var roleExist = await RoleManager.RoleExistsAsync(role.Name);
            if(!roleExist)
            {
                context.Roles.Add(role);
                context.SaveChanges();
            }
        }

        var devuser = await UserManager.FindByEmailAsync("[email protected]");
        await UserManager.AddToRoleAsync(devuser, "DEVELOPER");
        var work = await UserManager.FindByEmailAsync("[email protected]");
        await UserManager.AddToRoleAsync(work, "WORK");
    }