New Update Caused Some Errors

Hi there,

I just installed the Hustle update that appeared today and received a few errors specifically regarding opt-in forms created with the plugin; I have three opt-ins, and three errors are thrown at the top of the page (each identical):

"Warning: call_user_func() expects parameter 1 to be a valid callback, first array member is not a valid class name or object in /my/super/website/wp-content/plugins/hustle/opt-in.php on line 646".

Further, another warning has popped up on the homepage:

"Warning: Cannot modify header information - headers already sent by (output started at /my/super/website/wp-content/plugins/hustle/opt-in.php:646) in /my/super/website/wp-includes/pluggable.php on line 1210".

The "social sharing" feature appears fine, and I haven't used any "custom content" fields. After a quick search, I didn't see anything relating specifically to my issue. Disabling the plugin fixes it (obviously), but I'd like to have these running for my client. I do have daily restores set up with Snapshot, but would like to avoid using that if possible.

I did try re-saving the opt-ins, like you might re-save permalinks to update their structure when adding custom post types, just in case.

I am running multisite and have the plugin network enabled.

Thank you for any assistance you can provide!