Fixed errors

API/Controllers/SubjectsController.cs

@@ -40,7 +40,11 @@ namespace StudyLib.API.Controllers
                 return NotFound();
             }
 
+            if (subject.EditedBy == null)
+            {
                 subject.EditedBy = userId;
+            }
+
             _context.Entry(subject).State = EntityState.Modified;
             await _context.SaveChangesAsync();
 
@@ -55,6 +59,7 @@ namespace StudyLib.API.Controllers
                 return BadRequest();
             }
 
+            subject.EditedBy = null;
             _context.Entry(subject).State = EntityState.Modified;
             foreach (var comment in subject.Comments)
             {