CreateWidget node keeps reference to created widget on output pin

There was an issue in 4.16 (fixed in CL# 3437205) in which blueprint output pins would hold on to object references until the blueprint was destroyed. It appears that an additional fix may need to be applied to widget blueprints, as they're still holding on to a reference to the last widget created.

The reference will only be dropped when the blueprint containing the node is destroyed, or when it falls out of scope. Because of this, wrapping the create widget node in a function is a valid workaround.

1. Download attached project
2. Activate the "Add UMG label trigger n times
3. Type 'obj list class=redlabel_c' in the console to see that there are n ubojects created
4. Activate the Remove UMG label trigger
5. Type 'obj gc' in the console to trigger garbage collection on the now unreferenced widgets
6. Type 'obj list class=redlabel_c' again, and observe that there is still 1 widget that hasn't been collected