Remove old CodeMirrors when history load to fix #109 (#114)

Author: F-park

templates/components/challenge_area.html

@@ -63,6 +63,9 @@
    * @param {string} name - The name of the challenge.
    */
   function renderCodeArea(code_under_test, test_code, level, name) {
+    // delete old CodeMirrors to fix #109
+    document.querySelectorAll('.CodeMirror').forEach(el => el.remove());
+
     let confetti = new JSConfetti();
     let initTheme = localStorage.getItem('theme') === 'dark' ? "material-darker" : "default"
     let sharedCodeMirrorOptions = {