submit the correct form, fixes #1783

Closes #1783

Merge request studip/studip!1167

resources/assets/javascripts/bootstrap/forms.js

@@ -107,9 +107,7 @@ $(document)
         // Submit only if value has changed and either enter was pressed or
         // select was opened by click
         if (!is_default && shouldSubmit) {
-            $(this)
-                .closest('form')
-                .submit();
+            $(this.form).submit();
             return false;
         }
     });