Fix #107228: AttributeGroup.new crash when layer cannot be created

A layer cannot be created e.g. when creating a reserved-name attributes
with the wrong domain/type.

Caused by 101d04f41f [which just did not
check if the layer creation was successful].

This should go into 3.6 LTS as well.

Pull Request: https://projects.blender.org/blender/blender/pulls/118378

source/blender/makesrna/intern/rna_attribute.cc

@@ -399,6 +399,10 @@ static PointerRNA rna_AttributeGroup_new(
   CustomDataLayer *layer = BKE_id_attribute_new(
       id, name, eCustomDataType(type), AttrDomain(domain), reports);
 
+  if (!layer) {
+    return PointerRNA_NULL;
+  }
+
   if ((GS(id->name) == ID_ME) && ELEM(layer->type, CD_PROP_COLOR, CD_PROP_BYTE_COLOR)) {
     Mesh *mesh = (Mesh *)id;
     if (!mesh->active_color_attribute) {