The javadoc for File.listFiles() states:

An array of abstract pathnames denoting the files and directories in the directory denoted by this abstract pathname. The array will be empty if the directory is empty. Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

While rare, apparently this method does sometimes return null. We should add a null check at ResolverUtil:300 (in the 1.4.x branch). This also affects the head at ResolverUtil:255.