Fixed: allow to override child_classes and parent_classes with empty list
PEP8 cleanup with no funtional changes
Merge pull request #3385 from jrief/fix-placeholder_conf-empty-parent/child_classes

Fix placeholder conf empty parent/child classes
Added test for render_plugin in edit mode
fixed render_plugin when in edit mode
Merge pull request #3397 from ojii/3396-fix-render-plugin

Added test for render_plugin in edit mode
Check placeholder object exists before trying to access attributes

Fixes #3388