This is reproducible for me.

Using FTB Infinity 1.2.0

Crash Log:
http://pastebin.com/ewajdV5z

Let me know if you need any other details or if I should report this to Extra Utilities instead of RFTools.

I believe this is a problem with RFTools because from the crash log stack trace:

    at net.minecraft.item.ItemStack.func_82840_a(ItemStack.java:502)
    at com.rwtema.extrautils.gui.GuiFilingCabinet$ItemSorter.compare(GuiFilingCabinet.java:224)
    at com.rwtema.extrautils.gui.GuiFilingCabinet$ItemSorter.compare(GuiFilingCabinet.java:208)
    at java.util.TimSort.binarySort(Unknown Source)
    at java.util.TimSort.sort(Unknown Source)
    at java.util.Arrays.sort(Unknown Source)
    at java.util.Collections.sort(Unknown Source)
    at com.rwtema.extrautils.gui.GuiFilingCabinet.sortItems(GuiFilingCabinet.java:53)
    at com.rwtema.extrautils.gui.GuiFilingCabinet.func_73863_a(GuiFilingCabinet.java:200)
    at net.minecraft.client.renderer.EntityRenderer.func_78480_b(EntityRenderer.java:1061)

As you can see the crash happens from an uncaught NullPointerException when EU tries to compare one stack to the other, I think the responsibility for making compare work should be part of the item stack not EU, but I don't know that so I could totally be wrong to report this here.

It would be nice if you could also report this to EU. If only to be able to figure out what exactly EU is doing there so that I can know why this is failing. Because I see no rftools code in the stacktrace it is a bit hard to know where the null comes from

It is likely this is fixed with the somewhat more robust item handling code in rftools. If not reopen.