Fix backspace shortcut in codemirror

www/common/sframe-common-codemirror.js

@@ -416,8 +416,12 @@ define([
                     "Backspace": function () {
                         var cursor = doc.getCursor();
                         var line = doc.getLine(cursor.line);
-                        if (line.substring(0, cursor.ch).trim() === "") { editor.execCommand("indentLess"); }
+                        var beforeCursor = line.substring(0, cursor.ch);
-                        else { editor.execCommand("delCharBefore"); }
+                        if (beforeCursor && beforeCursor.trim() === "") {
+                            editor.execCommand("indentLess");
+                        } else {
+                            editor.execCommand("delCharBefore");
+                        }
 
                     },
                 });