You cannot edit the elements until the blueprint has been compiled.
Aside from that, a more important question is where your 300 actors come from. Are they manually placed in the level, or are they automatically generated? If it’s the former, you could get a populated array using the GetAllActorsOfClass method (assuming of course that your actors are of some specific class you have created for this purpose). If it’s the latter, you could add each actor to your empty array when they are spawned.