error when adding particles outside domain

modules/slimParticles/dgParticleTracker2D.cpp

@@ -1009,6 +1009,10 @@ void particleArray::addParticlesAtPoint(dgGroupCollection* _group, int M, double
   //Find group & element IDs
   auto r = _group->find(x0,y0,0);
   int iGroup = r.first, iElem = r.second;
+  if (r.first == -1){
+    Msg::Error("cannot add particle at point : %g %g", x0, y0);
+    return;
+  }
   
   //Seed particles
   for (int i=0; i<M; i++) {