code: @Nullable
      public Transition getTransitionAt(int index) {
          if (index < 0 || index >= mTransitions.size()) {
              return null;
          }
          return mTransitions.get(index);
      }
docs: Returns the child Transition at the specified position in the TransitionSet.
  
  @param index The position of the Transition to retrieve.
  @see #addTransition(Transition)
  @see #getTransitionCount()