Can't load buildmaps if they're in a folder with any other *.asset files

[virtushda]

I had to use a debugger to find this issue, but there's a suppressed null ref inside BundleBuildMap.CreateDataSources().
That null ref occurs when the folder contains any other *.asset file, the AssetDatabase.LoadAssetAtPath call is returning null because the type doesn't match.

Workaround:
Put build maps in their own folder.