MDL-27825 Improved check of themedesignermode to prevent errors if it isn't set

theme/index.php

@@ -153,7 +153,7 @@ if (!empty($CFG->enabledevicedetection) && empty($device)) {
             //obsoleted or broken theme, just skip for now
             continue;
         }
-        if (!$CFG->themedesignermode && $theme->hidefromselector) {
+        if (empty($CFG->themedesignermode) && $theme->hidefromselector) {
             // The theme doesn't want to be shown in the theme selector and as theme
             // designer mode is switched off we will respect that decision.
             continue;
@@ -194,4 +194,4 @@ if (!empty($CFG->enabledevicedetection) && empty($device)) {
 
 echo html_writer::table($table);
 
-echo $OUTPUT->footer();
+echo $OUTPUT->footer();