Fix crash when currentlySelectedIndices is empty.

Not sure how i triggered this, but found the issue in a core dump.

OpenMesh/OMPropertyModelT.cc

@@ -389,7 +389,7 @@ void OMPropertyModel<MeshT>::mouseEvent(QMouseEvent* _event)
         BaseObjectData* object;
         PluginFunctions::getPickedObject(node_idx, object);
 
-        if (object->id() == objectID_)
+        if (object->id() == objectID_ && !currentlySelectedIndices.empty())
         {
           OMPropertyVisualizer<MeshT>* viz = dynamic_cast<OMPropertyVisualizer<MeshT>*>(propertyVisualizers[currentlySelectedIndices.first().row()] );
           unsigned int primitiveId = viz->getClosestPrimitiveId(face_idx, hit_point);