No worries! Thanks for the report. I added a few checks to this part of the code and haven't been able to repro it, let me know if you see it again.
Getting this same problem on build 54. Rather constantly, in fact. Never on the first level but often within 3-4 stages. The inventory screen just does not respond to any clicks at all, and if you click to much you end up getting the "crash" a bit after you leave. Hovering functionality works fine though (item names and the glow around crafting spots).
Here's a save. Doom.wad.
Thanks! I just pushed an update (build 58) that totally refactors the crafting and related inventory code, so it should be more robust now. I can't repro the issue as described in the new build.
It looks like it's mostly fixed; I'm not getting it the majority of the time. I did run into one time though. It no longer ends in an out of bounds array access, no matter how much i tried to poke it, and unlike before while it stuck in the save file it didn't stick around when I reloaded from the beginning of the stage.