handlebar TypeError: Cannot convert undefined or null to object email handlebar firebase

This took me a while to understand, but you have to configure the template collection in extension settings.
1) Go to your firebase project, on the left sidemenu at the bottom should be a button "Extensions". Press it
https://gyazo.com/d336e5962ca3a02363252c48d98d3173
2) Choose "Trigger email" (If you can't see it you need to install the extension)
https://gyazo.com/79e4c57fc7829e4093193740544dcbac
3) Set the template collection
https://gyazo.com/5ce0a5ffabd7dcf901eaf69d2ada83b6