ensure labels are writed in quantify tag

src/output/masschroqml.cpp

@@ -267,11 +267,14 @@ MassChroQml::writeQuantify()
   _output_stream->writeStartElement("peptides_in_peptide_list");
   _output_stream->writeAttribute("mode", "post_matching");
   _output_stream->writeAttribute("ni_min_abundance", "0.8");
-  if(_sp_project.get()->getLabelingMethodSp().get() != nullptr)
+
+  LabelingMethod *p_labeling_method =
+    _sp_project.get()->getLabelingMethodSp().get();
+  if(p_labeling_method != nullptr)
     {
       QStringList isotope_label_ref;
       for(const Label *p_label :
-          _sp_project.get()->getLabelingMethodSp().get()->getLabelList())
+          p_labeling_method->getLabelList())
         {
           isotope_label_ref << p_label->getXmlId();
         }