Error message:

            modelBuilder.Entity<RoutingRule>()
                .HasOptional(m=>m.WorkflowStep)
                .WithMany()
                .HasForeignKey(m=>m.WorkflowStepFK);
                
                
                        //IsRequired
        public virtual int WorkflowStepFK { get; set; }
        public virtual WorkflowStep WorkflowStep { get; set; }
        
        
        \tSystem.Data.Entity.Edm.EdmAssociationType: : Multiplicity conflicts with the referential constraint in Role 'RoutingRule_WorkflowStep_Target' in relationship 'RoutingRule_WorkflowStep'. Because all of the properties in the Dependent Role are non-nullable, multiplicity of the Principal Role must be '1'.



Solution is: