Fixes: U4-6973 Given custom permissions user can create, but cannot save

src/Umbraco.Web/Models/Mapping/ContentModelMapper.cs

@@ -221,8 +221,16 @@ namespace Umbraco.Web.Models.Mapping
                 }
                 var svc = _userService.Value;
 
-                var permissions = svc.GetPermissions(UmbracoContext.Current.Security.CurrentUser, source.Id)
-                                              .FirstOrDefault();
+                var permissions = svc.GetPermissions(
+                    //TODO: This is certainly not ideal usage here - perhaps the best way to deal with this in the future is
+                    // with the IUmbracoContextAccessor. In the meantime, if used outside of a web app this will throw a null
+                    // refrence exception :(
+                    UmbracoContext.Current.Security.CurrentUser,
+                    // Here we need to do a special check since this could be new content, in which case we need to get the permissions
+                    // from the parent, not the existing one otherwise permissions would be coming from the root since Id is 0.
+                    source.HasIdentity ? source.Id : source.ParentId)
+                    .FirstOrDefault();
+
                 if (permissions == null)
                 {
                     return Enumerable.Empty<char>();