Still redundant…
A workaround for this issue in 4.15 ( in 4.14 I got an warning that I try to connect two null actors… ) is to use the Set Constrained Components in the blueprint and just leave the Constraint Actor 1 and Constraint Actor 2 empty.
However I need to use the Set Constrained Components twice! 
- In the Construction Script: To see the gizmos of the constraint that connect the two constrained actors.
- On Begin Play: To actually get the constraint to work.
Thing number three is that the Editor UI ( Constraint Actor 1 and Constraint Actor 2 ) of the Details panel is not updated at all and always displays None. I believe it should display the Constraint Actor 1 & 2 names after they were set in the Construction Script…
Is this buggy ?