handle nulls in normalizing ids

converter-CellDesigner/src/main/java/lcsb/mapviewer/converter/model/celldesigner/CellDesignerElementCollection.java

@@ -69,7 +69,7 @@ public class CellDesignerElementCollection {
       String sbmlId = getSbmlId(modelElement, true);
       if (sbmlIdByElement.get(sbmlId) == null) {
         String normalizedId = normalizeIdToCellDesigner(modelElement.getElementId());
-        if (!normalizedId.equals(modelElement.getElementId())) {
+        if (!Objects.equals(normalizedId, modelElement.getElementId())) {
           logger.warn(new LogMarker(ProjectLogEntryType.EXPORT_ISSUE, modelElement), "Invalid element id");
         }
         String id = "s_id_" + normalizeIdToCellDesigner(modelElement.getElementId());
@@ -296,6 +296,9 @@ public class CellDesignerElementCollection {
   }
 
   protected String normalizeIdToCellDesigner(String id) {
+    if (id == null) {
+      return null;
+    }
     return id.replaceAll("[:\\.-]", "_");
   }