AE2 calls this function when trying to set a pattern from REI, and it crashes dedicated servers. Would it be possible to soft-fail by returning ItemStack.EMPTY instead?

Update: after discussion with sharrte, you should try to return the best guess for the output, at least when there is only one output. I suggest returning the first stack. That will allow setting patterns from AE2.

Update: you should include the count in the return of getPreviewStacks too. To reproduce and test all of this, install TR + REI + AE2. If everything works as intended, it should be possible to shift-click the bronze ingot alloy smelter recipe into the pattern.