Just tested it, the reason that its crashing is that you are storing your levels inside the "levels" folder. Setting up a custom maps expects a dedicated folder for the campaign, check out how the "example" folder is set up to see how it needs to be done, or check out the tutorial I made for some other details.
Essentially you will need to add a new folder, probably the name of your campaign, say "MyHammerwatch." Inside this folder you will need an info.xml, and levels.xml as well as the icon.png and any additional folders for custom assets. So
MyHammerwatch
-levels (folder for levels)
-info.xml
-levels.xml
-icon.png
Your best bet is to copy and paste the "example" folder, rename the new one to whatever you want to call it, and then store all your levels inside the "levels" folder there. The "levels" folder inside the main editor directory is just for those menus and things, a custom campaign needs its own folder to exist in. The unpacked assets are mostly just for reference, all the editing takes place in that custom folder that you set up yourself. (You can also download the tutorial file and look at that, but the example folder should be sufficient).
