message: fetch null pointer, add meaningful log messages
diff: @@ -624,6 +624,18 @@ Bool_t AliTPCPreprocessorOffline::ProduceCombinedGainCalibration()
  // --- combine graphs
  grCombined = CombineGraphs(grOCDB, grThis);
  +
  + // --- check graph
  + if (!grCombined) {
  + if (TString(graphObj->GetTitle()).Atoi()) {
  + AliFatalF("%s could not be combined, object will be missing in the reconstruction. Considered to be critical", graphObj->GetName());
  + }
  + else {
  + AliWarningF("%s could not be combined, object will be missing in the reconstruction. Not considered to be critical", graphObj->GetName());
  + }
  + continue;
  + }
  +
  fGainArrayCombined->AddLast(grCombined);
  grCombined->SetMarkerColor(kMagenta);
  grCombined->SetLineColor(kMagenta);
  