Bug 81426 Package Explorer shows nothing after drill into working set, then show projects
I200412160800 - switch Package Explorer to working sets - context menu > Go Into a working set - Show > Projects -> view is empty -> view toolbar button 'Up to Working Sets' does not do anything
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/packageview/PackageExplorerPart.java
if (!isEssential(elements[i])) {
					for (int j = 0; j < filters.length; j++) {
						add= filters[j].select(this, root,
							elements[i]);
						if (!add)
							break;
					}
				}
				if (add)
					filtered.add(elements[i]);
			}
			return filtered.toArray();
		}
		
		/* Checks if a filtered object in essential (ie. is a parent that
		 * should not be removed).
		 */ 
		private boolean isEssential(Object object) {
			try {
				if (!isFlatLayout() && object instanceof IPackageFragment) {
					IPackageFragment fragment = (IPackageFragment) object;
					return !fragment.isDefaultPackage() && fragment.hasSubpackages();
				}
			} catch (JavaModelException e) {
				JavaPlugin.log(e);
			}
			return false;
		}
		
		protected void handleInvalidSelection(ISelection invalidSelection, ISelection newSelection) {