This method always returns null. In theory, renko bars doesn't have any in-progress bars.
The Wick or Tail gives additional information of the market movement during the drawing of any individual bar.
Example using a 5-pip Renko Bar and where market moves from 10 to 25:
Assuming that price movement was not simple one-way, but market moved from 10 -> 17 -> 13 -> 20 -> 17 -> 25,
the same RB with wicks will show the following 3 bars:
10 -> 15 no wick (null),
15 -> 20 with wick down to 13,
20 -> 25 with wick down to 17.
In case of rising Renko Bars: wicks can only be underneath of the bar-body.
In case of falling Renko Bars: wicks can only be above of the bar-body.