Unity 2D animated tiles library: IsSelected variable not working or not present within RuleTileEditor.cs

I have just tried downloading a library which will allow me to add animated tiles to unity. I have been following this tutorial:http://shadowpeakstudios.com/2018/10/22/unity-tutorial-animated-tiles/

It told me to download this library in order to work the animated tiles: https://github.com/Unity-Technologies/2d-extras

After this the tutorial told me to unzip the library and place it into my assets folder. It loaded into my assets folder and then got the following error:

Assetsd-extras-masterd-extras-master\Editor\Tiles\RuleTile\RuleTileEditor.cs(277,26): error  CS1061: 'ReorderableList' does not contain a definition for 'IsSelected' and no accessible extension  method 'IsSelected' accepting a first argument of type 'ReorderableList' could be found (are you  missing a using directive or an assembly reference?) 

Does anyone perhaps know why this error is occuring.