MDL-43069 assign: prevent JS error on grading table

Previously submission comments failed to expand if you hid
the file submissions column.

mod/assign/module.js

@@ -1,19 +1,22 @@
 M.mod_assign = {};
 
 M.mod_assign.init_tree = function(Y, expand_all, htmlid) {
-    Y.use('yui2-treeview', function(Y) {
-        var tree = new Y.YUI2.widget.TreeView(htmlid);
+    var treeElement = Y.one('#'+htmlid);
+    if (treeElement) {
+        Y.use('yui2-treeview', function(Y) {
+            var tree = new Y.YUI2.widget.TreeView(htmlid);
 
-        tree.subscribe("clickEvent", function(node, event) {
-            // We want normal clicking which redirects to url.
-            return false;
+            tree.subscribe("clickEvent", function(node, event) {
+                // We want normal clicking which redirects to url.
+                return false;
+            });
+
+            if (expand_all) {
+                tree.expandAll();
+            }
+            tree.render();
         });
-
-        if (expand_all) {
-            tree.expandAll();
-        }
-        tree.render();
-    });
+    }
 };
 
 M.mod_assign.init_grading_table = function(Y) {