[BP Nouveau] Prevent underscore errors if the activity post-form has been overriden

[imath]

I can imagine situations where people would want to completely override the BP Nouveau Activity Post form from their theme adding a buddypress/activity/post-form.php template in it.

In this case, underscore is throwing an error as the main element where is injected the Backbone Post Form is not available. I think it would be safer to avoid it checking for the main element before loading the Backbone views.

See attached patch.

[imath]

In 11993:

BP Nouveau: only load Activity form views if needed tag is available

Fixes #7769