This looks like the same issue Miah1377 reported after updating from an older version.
That function being referenced as the cause of the crash is trying to read a property that is assumed to be present on all weapons as of v2.62. I just released a sort of patch that stops it from causing the crash error, but the cause of the problem is that it appears that you are testing using a save file that was made prior to upgrading the plugin to version 2.62. The way the database for items is loaded is weird, and because this plugin wasn't reading notetags for locked wtypes prior to v2.62, the property that it saves onto weapons isn't present on your items database for the save file. The plugin (prior to 2.65) assumed that the property is always present, so it goes right to reading from it, which causes the crash error you posted when it finds the property as undefined.
As I said, v2.65, which was just uploaded a few hours ago will stop the crash (or it should anyway), but the actual fix is to start a new save-game file from which to test further.