code: @NullSafe
    public static Throwable getCause(Throwable throwable) {
      return (throwable != null ? throwable.getCause() : null);
    }
docs: Null-safe method returning the cause of the given {@link Throwable} object.  If the {@link Throwable} object
  reference is {@literal null} then this method returns {@literal null}.
  
  @param throwable {@link Throwable} object who's cause is returned.
  @return the cause of the given {@link Throwable} object.
  @see java.lang.Throwable#getCause()