Minor usability improvement for operations

TODO.FACTOR.txt

@@ -1,8 +1,6 @@
 + 0.87:
 
 - better error handling in early startup
-- some module operations don't work on module-links
-- list operations: what if nothing is selected?
 - slider needs to be modelized
 - overhaul models, set-model* is crap
 - color picker example

library/ui/tools/operations.factor

@@ -13,8 +13,12 @@ V{ } clone operations set-global
     <operation> operations get push-new ;
 
 M: operation invoke-command ( target operation -- )
-    dup command-quot swap operation-listener?
-    [ curry call-listener ] [ call ] if ;
+    2dup operation-predicate call [
+        dup command-quot swap operation-listener?
+        [ curry call-listener ] [ call ] if
+    ] [
+        2drop
+    ] if ;
 
 : modify-listener-operation ( quot operation -- operation )
     clone t over set-operation-listener?