Add null check to avoid future issues

src/main/java/edu/harvard/iq/dataverse/DatasetPage.java

@@ -2863,6 +2863,12 @@ public void sort() {
     public String refresh() {
         logger.fine("refreshing");
 
+        //In v5.14, versionId was null here. In 6.0, it appears not to be.
+        //This check is to handle the null if it reappears/occurs under other circumstances
+        if(versionId==null) {
+            logger.fine("versionId was null in refresh");
+            versionId = workingVersion.getId();
+        }
         //dataset = datasetService.find(dataset.getId());
         dataset = null;
         workingVersion = null; 
@@ -2872,10 +2878,9 @@ public String refresh() {
         DatasetVersionServiceBean.RetrieveDatasetVersionResponse retrieveDatasetVersionResponse = null;
 
         if (versionId != null) {
-            // versionId must have been set by now, in the init() method, 
-            // regardless of how the page was originally called - by the dataset
-            // database id, by the persistent identifier, or by the db id of
-            // the version. 
+            // versionId must have been set by now (see null check above), in the init()
+            // method, regardless of how the page was originally called - by the dataset
+            // database id, by the persistent identifier, or by the db id of the version.
             this.workingVersion = datasetVersionService.findDeep(versionId);
             dataset = workingVersion.getDataset();
         }