NPE in ResourceSetGlobalScopeProvider if no ResourceDescription for a Resource

I'm working with a ResourceSet that may contain non-Xtext resources.

ResourceSetGlobalScopeProvider.createScopeWithQualifiedNames() cannot handle that (NPE in line 71):

ISelectable compound = new ISelectable() {
	
	@Override
	public boolean isEmpty() {
		for (ISelectable description: resourceDescriptions) {
			// NPE for description
			if (!description.isEmpty())
				return false;
		}
		return true;
	}

Proposed fix: Add a nonNull filter to the resourceDescriptions iterator:

final Iterable<ISelectable> resourceDescriptions = Iterables.filter(
		Iterables.transform(resourceSet.getResources(),
				from -> this.resourceDecriptionProvider.getResourceDescription(from)),
		Objects::nonNull);

And when we're at it, converting the anonymous ISelectable within createScopeWithQualifiedNames() into a real class would support subclassing a lot.

Start at ResourceSetGlobalScopeProvider.createScopeWithQualifiedNames(), using a ResourceSet containing a non-Xtext resource to reproduce the null ResourceDescription failure. Ensure resources without descriptions no longer cause an NPE, and consider whether the anonymous ISelectable should become subclassable as proposed.