Check all PHPStorm errors (undefined variables and such) and resolve.
Merge remote-tracking branch 'upstream/development' into development
fixed Undefined index
Merge remote-tracking branch 'upstream/development' into development

Conflicts:
	core/class_functions.php
	core/ui/class_wpp_object_list_table.php
	default_api.php
fixed conflicts
Check all PHPStorm errors (undefined variables and such) and resolve. - Done
empty -> isset
wpi_ -> wpp_
fix conflicts
Merge remote-tracking branch 'upstream/development' into development
small fix
Merge pull request #21 from planv/development

Fixed undefined variables and wpi_ prefixes