add protection against missing database version files

smodels/experiment/databaseObj.py

@@ -295,6 +295,8 @@ def databaseVersion(self):
         """
         r = [x.databaseVersion for x in self.subs]
         for i, ri in enumerate(r):  # avoid repetitions
+            if ri == None:
+                r[i] = "unknown"
             for j, rj in enumerate(r[i+1:]):
                 if ri in rj:
                     r[i+j+1] = rj.replace(ri, "")