more robust startup;

--- a/src/Tools/jEdit/src/sledgehammer_dockable.scala	Sat Aug 17 22:15:45 2013 +0200
+++ b/src/Tools/jEdit/src/sledgehammer_dockable.scala	Sat Aug 17 22:27:41 2013 +0200
@@ -85,7 +85,8 @@
 
   private def query_provers()
   {
-    PIDE.session.protocol_command("Sledgehammer.provers")
+    if (PIDE.session.is_ready)
+      PIDE.session.protocol_command("Sledgehammer.provers")
   }
 
 
@@ -113,7 +114,7 @@
     PIDE.session.phase_changed += main_actor
     PIDE.session.global_options += main_actor
     Sledgehammer_Params.provers += main_actor
-    if (PIDE.session.is_ready) query_provers()
+    query_provers()
     handle_resize()
     sledgehammer.activate()
   }