Fixed getting empty selections.

system/dataset.py

@@ -183,7 +183,9 @@ class Dataset:
         backitems.sort()
         sorted_ids=[ backitems[i][1] for i in range(0,len(backitems))]
 
-        if indices != None:
+        # we use id as scipy-arrays return a new array on boolean
+        # operations
+        if id(indices) != id(None):
             return [sorted_ids[index] for index in indices]
         else:
             return sorted_ids