LINQ and EF6: The context cannot be used while the model is being created

I have an MVC 5 Application with EF6, and I tried my first LINQ query in a SignalR Hub. It runs when I click on a button on a Test view page.

But I get an exception at the query:

This query is my first individual code action in the MVC5 template with individual User Accounts by the template. I only created the model classes before.

If it helps: The context I'm using is the template ApplicationDbContext:IdentityContext<ApplicationUser>

My "OnModelCreating" method looks like this:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
        modelBuilder.Entity<Player>()
            .HasMany(m => m.MilitaryAccess)
            .WithMany();
        modelBuilder.Entity<Player>()
            .HasMany(m => m.FactionRelationship)
            .WithMany();
        base.OnModelCreating(modelBuilder);}

Thank you in advance!