Fix list of displayed nodes in Node Selection window

Only the root node was initially displayed, instead of the whole hierarchy. One had to click on the "Reset" button to make it appear, which isn't the expected behaviour.

Bug introduced in commit c6f79866.

src/interface/Node_select.cpp

@@ -168,7 +168,9 @@ void Node_select::set_container_names_rec(QTreeWidgetItem *current_node, Contain
         children = current_container->get_view_children();
     }
 
-    if( (load_view == false) || (children == NULL)) {
+    /* children cannot be NULL when accessed, because it means load_view==true
+     * and thus, children was assigned in the above condition block. */
+    if(load_view==false || children->empty()) {
         children = current_container->get_children();
     }