MDL-37397 Test for a valid select in formautosubmit

Ideally we shouldn't get into this situation, but in cases where the
generated select element is incorrectly configured, we should try and
handle it gracefully with a warning.

lib/yui/formautosubmit/formautosubmit.js

@@ -40,9 +40,17 @@ YUI.add('moodle-core-formautosubmit',
                 }
 
                 // Assign this select items 'nothing' value and lastindex (current value)
-                var thisselect = Y.one('select#' + this.get('selectid'));
-                thisselect.setData('nothing', this.get('nothing'));
-                thisselect.setData('startindex', thisselect.get('selectedIndex'));
+                if (this.get('selectid')) {
+                    var thisselect = Y.one('select#' + this.get('selectid'));
+                    if (thisselect) {
+                        if (this.get('nothing')) {
+                            thisselect.setData('nothing', this.get('nothing'));
+                        }
+                        thisselect.setData('startindex', thisselect.get('selectedIndex'));
+                    } else {
+                        Y.log("Warning: A single_select element was renderered, but the output is not displayed on the page.");
+                    }
+                }
             },
 
             /**