fix a bug on raid display.

when active or spare device list is null, eval() raises an exception.

interface/ri_tk_cmd.py

@@ -263,8 +263,11 @@ def list_to_list(list_from, var_from, var_to):
         itm = l_fr[idx]
         del l_fr[idx]
         display.var_dict[var_from].set(value=tuple(l_fr))
-        l_to = list(eval(display.var_dict[var_to].get()))
-        l_to.append(itm)
+        if display.var_dict[var_to].get():
+            l_to = list(eval(display.var_dict[var_to].get()))
+            l_to.append(itm)
+        else:
+            l_to = [ itm ]
         display.var_dict[var_to].set(value=tuple(l_to))
     
 def raid_raw_to_active():