The association between entities (with the key value, CreatedById:200) has been severed - Error

Question

While I try to delete a ProjectParticipant object from a Project object and call context.SaveChangesAsync(), I get this error:

ERROR: The association between entities ‘employee’ and ‘project’ with the key value ,CreatedById:200 has been severed but the relationship is either marked as ‘required’ or is implicitly required because the foreign key is not billable of the dependent/child entity should be deleted when a required relationship is severed, then setup the relationship to use cascade deletes.

Could someone help me fix this?

Project.cs

        public class Project
        {
            public Project()
            {
                ProjectParticipants = new HashSet<ProjectParticipant>();
            }
            public int Id { get; set; }

            public int CreatedById { get; set; }

            public Employee CreatedBy { get; set; }

            public virtual ICollection<ProjectParticipant> ProjectParticipants { get; set; }
        }
        }

Employee.cs

        public class Employee
        {
            public Employee()
            {
                Projects = new HashSet<Project>();
                ProjectParticipants = new HashSet<ProjectParticipant>();
            }
            public int Id { get; set; }
            public virtual ICollection<Project> Projects { get; set; }
            public virtual ICollection<ProjectParticipant> ProjectParticipants { get; set; }
        }

ProjectParticipant.cs

        public class ProjectParticipant
        {
            public ProjectParticipant()
            {
            }
            public int Id { get; set; }
            public int ProjectId { get; set; }
            public virtual Project Project { get; set; }
            public int EmployeeId { get; set; }
            public virtual Employee Employee { get; set; }
        }

ProjectDBContext.cs

        public class ProjectDBContext:DBContext
        { 
            public override OnModelCreating(ModelBuilder modelBuilder)
            {
                modelBuilder.Entity<Project>(entity => { 
                    ...
                    entity.HasOne(d => d.CreatedBy)
                    .WithMany(a => a.Projects)
                    .HasForeignKey(d => d.CreatedById)
                    .OnDelete(DeleteBehavior.ClientSetNull)
                    .HasConstraintName("FK_Project_Employee_CreatedBy");
                    ...
                });

                modelBuilder.Entity<ProjectParticipant>(entity => {
                    ...
                    entity.HasOne(d => d.Employee)
                    .WithMany(a => a.ProjectParticipants)
                    .HasForeignKey(d => d.EmployeeId)
                    .OnDelete(DeleteBehavior.Cascade)
                    .HasConstraintName("FK_ProjectParticipants_Employee");

                   entity.HasOne(d => d.Project)
                  .WithMany(a => a.ProjectParticipants)
                  .HasForeignKey(d => d.ProjectId)
                  .OnDelete(DeleteBehavior.Cascade)
                  .HasConstraintName("FK_ProjectParticipants_Project");
                    ...
                });

                modelBuilder.Entity<Employee>(entity => {
                    ...
                });
            }
        }