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: