[wp] avoid calling RefUsage from context menu

src/plugins/wp/GuiNavigator.ml

@@ -406,7 +406,7 @@ let model_varinfo :
   match item with
   | PLval(Some kf, _ , (Var x,NoOffset))
   | PTermLval(Some kf, _, _, (TVar {lv_origin=Some x},TNoOffset))
-    when button=1 ->
+    when button=1 && RefUsage.is_computed () ->
       begin
         let init = WpStrategy.is_main_init kf in
         let acc = RefUsage.get ~kf ~init x in

src/plugins/wp/RefUsage.ml

@@ -817,6 +817,7 @@ module S = State_builder.Option_ref(D)
 
 (* compute_usage is called once per project *)
 let usage () = S.memo compute_usage
+let is_computed () = S.is_computed ()
 
 (* ---------------------------------------------------------------------- *)
 (* --- API                                                            --- *)

src/plugins/wp/RefUsage.mli

@@ -41,3 +41,4 @@ val iter: ?kf:kernel_function -> ?init:bool -> (varinfo -> access -> unit) -> un
 val print : varinfo -> access -> Format.formatter -> unit
 val dump : unit -> unit
 val compute : unit -> unit
+val is_computed : unit -> bool