Returns the result that would be returned by {@link #join}, even
if this task completed abnormally, or {@code null} if this task
is not known to have been completed. This method is designed
to aid debugging, as well as to support extensions. Its use in
any other context is discouraged.
@return the result, or {@code null} if not completed
Returns the result that would be returned by {@link #join}, even if this task completed abnormally, or {@code null} if this task is not known to have been completed. This method is designed to aid debugging, as well as to support extensions. Its use in any other context is discouraged.
@return the result, or {@code null} if not completed