Can't save new record into table using C# and Entity Framework because of NullReference error

Here is my problem:

My button click event handler:

protected void add_btn_Click(object sender, EventArgs e)
{
        motorok m = new motorok();
        m.marka = t1.Text;
        m.tipus = t2.Text;

        if (t3.Text.Length > 0)
        {

            m.evjarat = Convert.ToInt32(t3.Text);
        }

        if (t4.HasFile)
        {
            m.kep = t4.FileName;
        }

        sqlchannel.SaveNewMotor(m);
}

My method to save the data in the database:

public static void SaveNewMotor(motorok m)
{
        MOTORAB_MODEL.motorok.Add(m);
        MOTORAB_MODEL.SaveChanges();
}

My table:

CREATE TABLE [dbo].[motorok] 
(
    [id]      INT           IDENTITY (1, 1) NOT NULL,
    [marka]   VARCHAR (50)  NOT NULL,
    [tipus]   VARCHAR (100) NOT NULL,
    [evjarat] INT           NULL,
    [kep]     VARCHAR (100) NULL,


    PRIMARY KEY CLUSTERED ([id] ASC)
);

Error: the MOTORAB_MODEL.SaveChanges() call throws a System.NullReferenceException

I think the problem has to be with the IDENTITY somehow but I can't figure out why. The method gets the object correctly with every value I want. In the DB, in my 'motorok' table the id column is set to IDENTITY and it is an integer. A select works perfectly so the database connection is fine.

Can you please help me? I've been googling for days now.

Edit:

Screenshot of the error

Edit 2:

MOTORAB_MODEL (motorab.mdf is my database file):

private static MOTORAB MOTORAB_MODEL;

static sqlchannel()
{
    MOTORAB_MODEL = new MOTORAB();
}

I haven't override SaveChanges().