Greetings.
Sorry for the problem here, but the error code you posted is just what I needed to track this down. The problem here is that I believe you are using an old save to test your project, before you installed the new version of this plugin. That function causing the error is checking an array that should be present on all weapons as of the 2.62 update - specifically, even weapons that do not prevent the use of other wtypes will still have this value, it's simply blank, so it doesn't cause this error.
The simplest way to prevent this error is to start a new save-game in your project to test. I have just uploaded version 2.65, which will also correct this issue (causing crashes isn't good, even if the reason isn't exactly the plugin's fault).