MC Version 1.11.0 on forge 2188 with RFTools 1.1x-5.72
Tried to make a wither skeleton spawner. Added RF to the spawner and 3 matter beamers. Placed bones, netherrack, and wheat. Flicked the first lever and the system crashed. Also is attached a screen shot of the spawner gui. Did not show items needed.

PM
Description: Ticking block entity

java.lang.NullPointerException: Ticking block entity
at mcjty.lib.tools.ItemStackTools.isEmpty(ItemStackTools.java:71)
at mcjty.rftools.blocks.spawner.SpawnerConfiguration$MobSpawnAmount.match(SpawnerConfiguration.java:386)
at mcjty.rftools.blocks.spawner.SpawnerTileEntity.addMatter(SpawnerTileEntity.java:142)
at mcjty.rftools.blocks.spawner.MatterBeamerTileEntity.checkStateServer(MatterBeamerTileEntity.java:106)
at mcjty.rftools.blocks.spawner.MatterBeamerTileEntity.func_73660_a(MatterBeamerTileEntity.java:54)
at net.minecraft.world.World.func_72939_s(World.java:1827)
at net.minecraft.world.WorldServer.func_72939_s(WorldServer.java:621)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:720)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:624)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:149)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:482)
at java.lang.Thread.run(Thread.java:745)

Fixed next release (both the crash and the ability to spawn). You might have to renegenerate the config file