Sending Entity sources that are not Nodes gives notices.

I've implemented a Commerce Order Source class based on the work done in this issue #1890718: Provide a default Entity source class. When I send a mail I get the following errors:

Notice: Undefined index: #node in template_preprocess_simplenews_newsletter_body() (line 2525 of public_html/sites/all/modules/modified/simplenews/simplenews.module).
Notice: Trying to get property of non-object in template_preprocess_simplenews_newsletter_body() (line 2525 of public_html/sites/all/modules/modified/simplenews/simplenews.module).

I think the solution is to look for #entity or # . $entity_type instead of #node.

I'll write a patch for this later, but there may be more tweaks to make