Fixes the constraint violation error in DestroyDataseCommand, when th…

…e doomed

dataset has an explicitly assigned thumbnail DataFile.

src/main/java/edu/harvard/iq/dataverse/engine/command/impl/DestroyDatasetCommand.java

@@ -53,12 +53,17 @@ protected void executeImpl(CommandContext ctxt) throws CommandException {
                 this,  Collections.singleton(Permission.DeleteDatasetDraft), doomed);                
         }
 
+        // If there is a dedicated thumbnail DataFile, it needs to be reset
+        // explicitly, or we'll get a constraint violation when deleting:
+        doomed.setThumbnailFile(null);
         final Dataset managedDoomed = ctxt.em().merge(doomed);
 
 
         List<String> datasetAndFileSolrIdsToDelete = new ArrayList<>();
         // files need to iterate through and remove 'by hand' to avoid
-        // optimistic lock issues....        
+        // optimistic lock issues... (plus the physical files need to be 
+        // deleted too!)
+
         Iterator <DataFile> dfIt = doomed.getFiles().iterator();
         while (dfIt.hasNext()){
             DataFile df = dfIt.next();