Bug 11748 switch to new DecoratorManager API
From the workbench change notes: API changes IWorkbenchPartSite.getDecoratorManager() - As the DecoratorManager has proven to be required for more than just IWorkbenchPartSites we have moved getDecoratorManager() to IWorkbench and deprecated the getDecoratorManager() in IWorkbenchPartSite. The one in IWorkbenchPartSite will be removed after the other Eclipse components have had a chance to switch to the new API.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/typehierarchy/TypeHierarchyViewer.java
* Returns true if the hierarchy contains elements. Returns one of them
	 * With member filtering it is possible that no elements are visible
	 */ 
	public Object containsElements() {
		TypeHierarchyContentProvider contentProvider= getHierarchyContentProvider();
		if (contentProvider != null) {
			Object[] elements= contentProvider.getElements(null);
			if (elements.length > 0) {
				return elements[0];
			}
		}
		return null;
	}	
	
	/**
	 * Returns true if the hierarchy contains elements. Returns one of them
	 * With member filtering it is possible that no elements are visible
	 */ 
	public IType getTreeRootType() {
		TypeHierarchyContentProvider contentProvider= getHierarchyContentProvider();
		if (contentProvider != null) {		
			Object[] elements=  contentProvider.getElements(null);
			if (elements.length > 0 && elements[0] instanceof IType) {
				return (IType) elements[0];
			}
		}
		return null;
	}