slapos_cloud: fix default image usage

master/bt5/slapos_cloud/SkinTemplateItem/portal_skins/slapos_cloud/InstanceTree_getDefaultImageAbsoluteUrl.py

@@ -6,11 +6,14 @@ image_url = "gadget_slapos_panel.png"
 release = portal.portal_catalog.getResultValue(
     portal_type="Software Release",
     url_string=url_string,
+    follow_up__uid=context.getFollowUpUid()
 )
 if release is not None:
   software_product = release.getAggregateValue(
     checked_permission='Access contents information')
   if software_product is not None:
-    image_url = software_product.getDefaultImageAbsoluteUrl()
+    product_image_url = software_product.getDefaultImageAbsoluteUrl()
+    if product_image_url:
+      image_url = product_image_url
 
 return image_url