BUG: Correct access to empty vector of class field index in unsupervised

Modules/Applications/AppClassification/app/otbTrainVectorClassifier.cxx

@@ -65,7 +65,7 @@ protected:
   {
     m_FeaturesInfo.SetClassFieldNames( GetChoiceNames( "cfield" ), GetSelectedItems( "cfield" ) );
 
-    if( m_FeaturesInfo.m_SelectedCFieldIdx.empty() )
+    if( m_FeaturesInfo.m_SelectedCFieldIdx.empty() && GetClassifierCategory() == Supervised )
       {
       otbAppLogFATAL( << "No field has been selected for data labelling!" );
       }

Modules/Applications/AppClassification/include/otbTrainVectorBase.h

@@ -125,7 +125,7 @@ protected:
     {
       m_SelectedCFieldIdx = selectedCFieldIdx;
       // Handle only one class field name, if several are provided only the first one is used.
-      m_SelectedCFieldName = cFieldNames[selectedCFieldIdx.front()];
+      m_SelectedCFieldName = selectedCFieldIdx.empty() ? cFieldNames.front() : cFieldNames[selectedCFieldIdx.front()];
     }
   };