Fix for null pointer in settings manager buildTree:

Fixes https://github.com/geonetwork/core-geonetwork/issues/681
Fix for css and less compilation.

It was found that the WRO4j importer doesn't handle /* ... */ comment blocks correctly and results in a corrupted css file after merging files with /*...*/ comment blocks.  This fix removes these blocks before performing the import to fix this issue.