Fix T45891: Crash generating previews

source/blender/blenkernel/intern/group.c

@@ -156,6 +156,7 @@ Group *BKE_group_copy(Group *group)
 	BLI_duplicatelist(&groupn->gobject, &group->gobject);
 
 	/* Do not copy group's preview (same behavior as for objects). */
+	groupn->preview = NULL;
 
 	if (group->id.lib) {
 		BKE_id_lib_local_paths(G.main, group->id.lib, &groupn->id);

source/blender/blenkernel/intern/object.c

@@ -1562,6 +1562,7 @@ Object *BKE_object_copy_ex(Main *bmain, Object *ob, bool copy_caches)
 	}
 
 	/* Do not copy object's preview (mostly due to the fact renderers create temp copy of objects). */
+	obn->preview = NULL;
 
 	return obn;
 }