Fix bug in sandbox's Math.random()

lib/sandbox.js

@@ -54,7 +54,7 @@ class SandboxDate {
 
 function random() {
   if (!state.rng)
-    state.rng = seedrandom(sessionContext.timestamp);
+    state.rng = seedrandom(state.sessionContext.timestamp);
 
   return state.rng();
 }