erp5_ci_slideshow: fix no related object raises on WebPage_viewAsWeb

bt5/erp5_ci_slideshow/SkinTemplateItem/portal_skins/erp5_ci_slideshow/WebPage_viewAsWeb.py

@@ -60,6 +60,7 @@ def getThemeFromFirstFollowUpProduct(reference):
       if category.find(product_match_string) > 1:
         relative_url = category.split("follow_up/")[1]
         category_object = portal.portal_catalog(relative_url=relative_url,limit=1)
+        if len(category_object) > 0:
           category_title = category_object[0].getTitle()
           theme = category_title.split(software_match_string)[0].lower()