I only expected C# Method to be affected from the .NET Core changes. If there something else had change, it´s definitely helpful to know about these kind of stuff.
My suggestion: Do like Aras do. They most of the time use: var inn = this.getInnovator(); in JS Methods. Maybe check some existing Methods how Aras calls the Innovator object.
In addition, don´t use top.aras. Top is deprecated since many versions. In the past it most of the time didn´t make any trouble, but maybe today it will.
Thanks AngelaIp, that command gives the same error. We've opened up a bug report on this one with Aras as we've tried just about every variation of it from the Programmer's guide and nothing seems to be working. OOTB methods don't see to have something exactly the same, but those that have similar commands seem to be referring to the same commands, so stumped on this..
var innovator = new Innovator();
var pr = innovator.newItem("PR","add")
var pr = new Item();
var pr = this.newItem("PR","add");
var myInnovator = this.getInnovator();
var pr = myInnovator.newItem("PR","add");
var pr = this.newItem("PR","add");
var pr = aras.newItem("PR","add");
Everything looks perfectly fine (your code - not Innovator 15,.cause I will never forgive Aras they dared to change the color of the loading spinner into PURPLE)..
And you obviously don´t do this the first time, cause then you would not come up with TGV actions at all.
Does your Method works when called from an innocent Form button?
Maybe entry point for Aras object has changed, for TGV for some reason I used "window.aras" in the past:
var inn = this.getInnovator();
const aras = window.aras;
But don´t spend too much time testing on your own. I am curious myself about the official solution by Aras.
Don´t get confused by to many variants 
this.getInnovator() always fails on Form button event.
var inn = new Innovator() works
var inn = aras.newIOMInnovator();
The last one is yet another variant. And when you already busy writing to Aras - ask them if they can make an overview of them. Would be perfect topic for a blog article. :-D
Fixed now. Aras support recommended use of the aras.newIOMInnovator();. From there, was missing a separate command for defining the "this". Haven't replaced the "top" command yet, but this works from either a form button or the TGV.
var inn = aras.newIOMInnovator();
var thisItem = parent.thisItem;
var pr = inn.newItem("PR","add");
pr.setProperty("_impactedproject", thisItem.getID());
top.aras.uiShowItemEx(pr.node, "tab view", true, true);
