This was reported on the RFTools issue tracker by @ReikaKalseki :
McJtyMods/RFTools#441

I'm not exactly sure what is going on but it seems like GenLayerRiverMixWrapper is calling the superconstructor for GenLayerRiverMix with the two generator chain parameters set to null. I have no idea why this only causes a problem in rftools dimensions but to me it seems suspicious that you are passing null here since many functions in GenLayerRiverMix depend on these values being set. Like for example GenLayerRiverMix:initWorldgenSeed.

Advice wanted. Thanks

Short answer: GenLayerRiverMixWrapper is not a GenLayerRiverMix; it's a completely different routine. It has to masquerade as GenLayerRiverMix to be handed to vanilla routines. I could stuff dummy routines in the parent but they wouldn't be connected to what's going on.

What is it you need to do with the vanilla parameters?

The thing is its initWithWorldSeed(long seed) function is being called, which makes direct reference to the null internal fields in the parent object.

OK, I'll pass some dummy genlayers for that.