[FIX] web_editor: prevent crash with FieldTextHtmlSimple

If the method commitChange from the widget FieldTextHtmlSimple was
called in mode=readonly, a traceback was displayed.  This is not very
common, because most save operations are done in mode=edit, but it can
happen in readonly mode: when the user clicks on an action button, then
the form view is saved, if necessary.

To reproduce this bug, just open a form view for res.users, and click on
the Change Password button.

addons/web_editor/static/src/js/backend.js

@@ -39,7 +39,7 @@ var FieldTextHtmlSimple = basic_fields.DebouncedField.extend({
      */
     commitChanges: function () {
         // switch to WYSIWYG mode if currently in code mode to get all changes
-        if (config.debug) {
+        if (config.debug && this.mode === 'edit') {
             var layoutInfo = this.$textarea.data('layoutInfo');
             $.summernote.pluginEvents.codeview(undefined, undefined, layoutInfo, false);
         }

addons/web_editor/static/tests/web_editor_tests.js

@@ -121,4 +121,31 @@ QUnit.test('field html_frame widget', function (assert) {
     form.destroy();
 });
 
+QUnit.test('field htmlsimple does not crash when commitChanges is called in mode=readonly', function (assert) {
+    assert.expect(1);
+
+    var form = testUtils.createView({
+        View: FormView,
+        model: 'mass.mailing',
+        data: this.data,
+        arch: '<form string="Partners">' +
+                '<header>' +
+                    '<button name="some_method" class="s" string="Do it" type="object"/>' +
+                '</header>' +
+                '<sheet>' +
+                    '<field name="body"/>' +
+                '</sheet>' +
+            '</form>',
+        res_id: 1,
+        intercepts: {
+            execute_action: function () {
+                assert.step('execute_action');
+            }
+        },
+    });
+
+    form.$('button:contains(Do it)').click();
+    form.destroy();
+});
+
 });