I am running into this as well. This notice appeared after updating the plugin (Contact Form Builder for WordPress – Conversion Tools by HubSpot) to version 7.0.3. This notice only appears when this plugin is active. Changing themes does not make a difference.
I am having this same problem. I do not have any JavaScript errors in my console. I have double checked my API keys and settings in WooCommerce and Stripe. I am at a loss here. Does anyone have any suggestions?
I ran into this issue as well. This fixed an issue that I was having on the front end when a search query was submitted blank in addition to the error on post filter in the admin. Thanks for the fix. It seems like this is something that should easily be fixed by the admin in an update…