WidgetLookup should set workbenchpart as parent if shell is null

if (activeWorkbenchParentShell == null || !activeWorkbenchParentShell.equals(activeShell)){
			if (activeShell != null){
				logger.debug("Setting active shell with title \"" + WidgetHandler.getInstance().getText(activeShell) + "\" as the parent");
				control = activeShell;	
			}
		}