You can’t get components that you added to a Blueprint Asset in C++. Well, actually you can - but it’s not a good workflow or design.
The best approach is to make a new C++ class and add your extra component there, that way you know it’s always valid and you can access it. This may not be neccesary however, as you may be able to use the Capsule Collision that is already part of the Character class for your collision tests (unless you need to check against different collision channels).