In the very near future, we are moving the Editor code to module base ES6 which means that PCUI won’t be exposed as window.pcui
We will however, be exposing it under a global namespace such as window.pcuix or window.pcuiexternal
Once we have firmed up plans, we let you know as soon as possible
CC @Elliott
The name we have chosen is window.pcuiExternal
This is expected to change pretty soon, this week or next
Change of plans 
We just had an internal chat about this and there are bigger problems going along this line that affects Editor Plugins official support.
For the short term, we will be sticking with window.pcui until we finalised these plans