How to get the OWIN context on ASP.Net Identity?

I'm trying to get the DbContext from the current Owin context, so I can use a single context on my application, however, I'm getting a NullReferenceException.

I can access UserManager and RoleManager:

private ApplicationUserManager _userManager;
    public ApplicationUserManager UserManager
    {
        get
        {
            return _userManager ?? HttpContext.GetOwinContext().GetUserManager<ApplicationUserManager>();
        }
        private set
        {
            _userManager = value;
        }
    }

They're configured how they came by default in the Identity sample project:

app.CreatePerOwinContext(ApplicationDbContext.Create);
app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
app.CreatePerOwinContext<ApplicationRoleManager>(ApplicationRoleManager.Create);
app.CreatePerOwinContext<ApplicationSignInManager>(ApplicationSignInManager.Create);

But trying to get the context to use it directly:

ApplicationDbContext context = HttpContext.GetOwinContext().Get<ApplicationDbContext>();

It always returns null on my controller. How can I get the current DbContext from the Owin context?

EDIT:

I was creating a new context to use with my generic repository

public AdminController()
    {
        AppContext = new ApplicationDbContext();
        this.repoProyects = new GenericRepository<Proyect>(AppContext);
    }

But it was creating a problem with entities being referenced from multiple contexts, so I'm trying to get the current Owin context like this:

public AdminController()
    {
        this.AppContext = HttpContext.GetOwinContext().Get<ApplicationDbContext>();
        this.repoProyects = new GenericRepository<Proyect>(AppContext);
    }

HttpContext is always null from here, so I don't know how to get the context to pass it to my class.