fix possible deref of nullptr error message

Author: EmilGorm

PWGCF/GenericFramework/Core/FlowContainer.cxx

@@ -916,8 +916,12 @@ TProfile* FlowContainer::GetRefFlowProfile(const char* order, double m1, double
     delete tempprof;
   }
   delete rhSubset;
-  retpf->RebinX(nBins);
-  return retpf;
+  if (retpf) {
+    retpf->RebinX(nBins);
+    return retpf;
+  } else {
+    return nullptr;
+  }
 };
 
 //{2} particle correlations