Migrations The ALTER TABLE statement conflicted with the FOREIGN KEY constraint

I want to créate a new table ClientsGroup that can relate to a table that allready exists and has data Client, and when I run the migrations it gives me the error 

The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_Clients_ClientsGroups_clientsGroupId". The conflict occurred in database "gino", table "dbo.ClientsGroups", column 'clientsGroupId'.

I have read several answers on fórums but I can not understand for example one person wrote this

Update Client set clientsGroupId = NULL

or WITH NOCHECK wich I don't know how to use in migrations

this is the migration code

    public partial class _4 : Migration
    {
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.AddColumn<int>(
                name: "clientsGroupId",
                table: "Clients",
                type: "int",
                nullable: false,
                defaultValue: 0);

            migrationBuilder.CreateTable(
                name: "ClientsGroups",
                columns: table => new
                {
                    clientsGroupId = table.Column<int>(type: "int", nullable: false)
                        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
                    name = table.Column<string>(type: "nvarchar(max)", nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_ClientsGroups", x => x.clientsGroupId);
                });

            migrationBuilder.CreateIndex(
                name: "IX_Clients_clientsGroupId",
                table: "Clients",
                column: "clientsGroupId");

            migrationBuilder.AddForeignKey(
                name: "FK_Clients_ClientsGroups_clientsGroupId",
                table: "Clients",
                column: "clientsGroupId",
                principalTable: "ClientsGroups",
                principalColumn: "clientsGroupId",
                onDelete: ReferentialAction.Cascade);
        }

        protected override void Down(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.DropForeignKey(
                name: "FK_Clients_ClientsGroups_clientsGroupId",
                table: "Clients");

            migrationBuilder.DropTable(
                name: "ClientsGroups");

            migrationBuilder.DropIndex(
                name: "IX_Clients_clientsGroupId",
                table: "Clients");

            migrationBuilder.DropColumn(
                name: "clientsGroupId",
                table: "Clients");
        }
    }