Fix H17 regression where it would crash when deleting/reloading a custom expression function before its first evaluation.

Fix H17 regression where it would crash when deleting/reloading a custom expression function before its first evaluation.

Fix H17 regression where it would crash when deleting/reloading a custom expression function before its first evaluation.

Fix H17 regression where it would crash when deleting/reloading a custom expression function before its first evaluation.

Fix H17 regression where it would crash when deleting/reloading a custom expression function before its first evaluation.