Interesting. According to Lucas, this can happen if your game is set to run as admin, but the P3D editor isn't. Try running the P3D Editor as admin and try again.
Also, looking at the script some more, I'm seeing a couple of issues with the script.
While this shouldn't be causing any crashing since both of them use the same locator type, you're using what the game normally uses as car waypoints as item locators, which probably isn't the best thing to do. Go open Level 1's m6.p3d, and copy the item locators (m6_videogamesX) into one of the P3Ds you have, and rename it to whatever you want (like item_X). Alternatively, you can load the m6.p3d into the script and use he m6_videogame locators instead for AddCollectible, but it's much more convenient to have everything in one P3D file.
You're also using the wrong skeleton name for the bonebox. While bonebox.p3d is what you want to load, the skeleton is named differently (not really your fault. Just Radical's odd inconsistency).
In short, just change this:
AddCollectible("m5_van_path4","bonebox");
to:
AddCollectible("m5_van_path4","bonestorm");
That should fix the crashing issue. I would highly recommend you copy the m6_videogames from Level 1's Mission 6 P3D file though and use that for AddCollectible instead of the "m5_van_pathX" locators.