How to add a foreign key and navigation property to a composite primary key entity?

Question

I have an entity with composite primary key, and I want to add a additional foreign key and its navigation property to it.

But I always get an error:

The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_TaskRelations_Projects_ProjectId". The conflict occurred in database "Database", table "dbo.Projects", column 'Id'

This occurs after running update-database.

This is my entity:

public class TaskRelation
{
    public long PrecedingTaskId { get; set; }
    public long FollowingTaskId { get; set; }
    public long ProjectId { get; set; }
    [ForeignKey("PrecedingTaskId")]
    [JsonIgnore]
    public MainTask PrecedingTask { get; set; }
    [ForeignKey("FollowingTaskId")]
    [JsonIgnore]
    public MainTask FollowingTask { get; set; }
    [ForeignKey("ProjectId")]
    [JsonIgnore]
    public ProjectBase Project { get; set; }
}

My fluent Api is like below:

modelBuilder.Entity<TaskRelation>().HasKey(c => new { c.PrecedingTaskId, c.FollowingTaskId });
modelBuilder.Entity<TaskRelation>()
            .HasOne(ta => ta.FollowingTask)
            .WithMany(t => t.TaskAssignments)
            .HasForeignKey(ta => ta.FollowingTaskId)
            .OnDelete(DeleteBehavior.Restrict);

Would you please give me some advice about this problem? Thank you!