code: public StrBuilder insert(final int index, final Object obj) {
          if (obj == null) {
              return insert(index, nullText);
          }
          return insert(index, obj.toString());
      }
docs: Inserts the string representation of an object into this builder.
  Inserting null will use the stored null text value.
  
  @param index  the index to add at, must be valid
  @param obj  the object to insert
  @return this, to enable chaining
  @throws IndexOutOfBoundsException if the index is invalid