ensure accessor is not empty

HLT/Trigger/TrigControl/TrigServices/src/WebdaqHistSvc.cxx

@@ -221,7 +221,7 @@ T* WebdaqHistSvc::getHist_i(const std::string& id, const size_t& /*ind*/, bool q
   ATH_MSG_DEBUG("Getting histogram " << id);
 
   tbb::concurrent_hash_map<std::string, THistID>::const_accessor accessor;
-  if (!m_hists.find(accessor, id)) {
+  if (!m_hists.find(accessor, id) or accessor.empty()) {
     if (!quiet) ATH_MSG_ERROR("could not locate Hist with id \"" << id << "\"");
     return nullptr;
   }