For the Call to undefined method Flarum\Foundation\Application::resolving() error, you must add the following at the beginning
use Illuminate\Container\Container
and then change function (Application $app) to function (Container $app), as this states that the app helper is instances a different class starting from beta 14. However, for some reason the less variable is registered as expected. One may also comment/delete the whole failing function and define the variable in the vendor/starsrivers/flarum-mathren/less/forum/copy-tex.less file directly.
This, however, triggers another error (Source map version 4 is unsupported. Supported only 3.), which was introduced by this change made by @StarsRiver. We can sidestep that problem by moving/deleting the vendor/starsrivers/flarum-mathren/js/dist/admin.js.map file.
But then, I get a app.extensionSettings is undefined in the admin page, this seems to be related to the admin page refactors in beta 15, which I haven't investigated yet.
I hope this helps to get the extension in a working state again.
