TypeError: Cannot read properties of undefined (reading 'template_options')

[cviebrock]

After recently upgrading a site from Drupal 9 to 10, and this package from 2.9.1645644008 to 3.5.1709296836, we're having issues with editing mailings. Specifically, when you click on the "TEST" button to send a test email, the modal pops up, but none of the form elements on the modal are clickable: the preview buttons aren't clickable, I can't change the test email destination, nor click on the Send test button).

When the modal opens, I get the following error in Chrome dev tools in the following file https://example.ca/sites/default/files/civicrm/persist/contribute/dyn/angular-modules.8d867be56648f37143ace17e16449636.js

angular-modules.8d867be56648f37143ace17e16449636.js:3651  TypeError: Cannot read properties of undefined (reading 'template_options')
    at b.isSplit (angular-modules.8d867be56648f37143ace17e16449636.js:4227:22)
    at fn (eval at compile (angular.js:16548:15), <anonymous>:4:217)
    at m.$digest (angular.js:19262:34)
    at m.$apply (angular.js:19630:13)
    at cmd.execute (angular-modules.8d867be56648f37143ace17e16449636.js:3651:24)
    at HTMLAnchorElement.<anonymous> (knockout-latest.js:100:264)
    at HTMLAnchorElement.dispatch (jquery.js:5135:27)
    at p.handle (jquery.js:4939:28)
(anonymous) @ angular.js:15697
(anonymous) @ angular.js:11956
$digest @ angular.js:19288
$apply @ angular.js:19630
cmd.execute @ angular-modules.8d867be56648f37143ace17e16449636.js:3651
(anonymous) @ knockout-latest.js:100
dispatch @ jquery.js:5135
p.handle @ jquery.js:4939

Any ideas what's happening? I'm not sure this is an issue with my install, or the upgrade, or if I missed a step.

[cviebrock]

Quick follow-up: I've tried installing earlier versions of the extension from the latest 3.5 all the way back to 2.11. I don't see the above error message in 3.4 and before, but all the controls in the modal are still disabled and unclickable.

[cviebrock]

Further follow-ups:

1. I can tab through the fields in the modal and update the test email field, for example, but using the cursor to click on things doesn't work.
2. I think the issue can be traced back to this function in ang/crmMosaico/Variants.js:

      // isSplit(mailing): Determine if there are -any- split/variant fields.
      // isSplit(mailing, field): Determine if a -specific- field has variations.
      isSplit: function isSplit(mailing, field) {
        if (!mailing.template_options || !mailing.template_options.variants) {
          return false;
        }
        if (field) {
          return (field in mailing.template_options.variants[0]);
        }
        const nonEmpties = _.filter(mailing.template_options.variants, (v) => !_.isEmpty(angularObj(v)));
        return nonEmpties.length > 0;
      }

For some reason that function is called with mailing being undefined, and then it barfs. I'm not sure if it's as simple as adding a check for that in the function itself, or if something else is happening upstream.