proper GUI_Thread context (cf. 7e0d3da6e6d8);

--- a/src/Tools/jEdit/src/isabelle_sidekick.scala	Thu Dec 04 20:56:38 2014 +0100
+++ b/src/Tools/jEdit/src/isabelle_sidekick.scala	Thu Dec 04 21:28:35 2014 +0100
@@ -83,7 +83,7 @@
 
     // FIXME lock buffer (!??)
     val data = Isabelle_Sidekick.root_data(buffer)
-    val syntax = Isabelle.buffer_syntax(buffer)
+    val syntax = GUI_Thread.now { Isabelle.buffer_syntax(buffer) }
     val ok =
       if (syntax.isDefined) {
         val ok = parser(buffer, syntax.get, data)