Attaching Component at runtime

I have a class that has a two dimensional array for storing pointers to UMyField Objects. I have to fill that array in the BeginPlay() method, because I don’t have the object’s bounds or properties from the editor in the constructor. The Engine crashes when I try to attach the component to the root component. Here’s the code:
UMyField* NewField = NewObject(UMyField::StaticClass());
what do I have to do?