PHP errors when previewing new node

When an new node with a makemeeting field is created, the node id (nid) doesn't exist yet.
This causes php errors when previewing this new node, because the nid is needed for the makemeeting_answers_form_nid form id.

Errors:

• Notice: Undefined index: makemeeting_answers_form_ in drupal_retrieve_form()...
• Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'makemeeting_answers_form_' not found or invalid function name in drupal_retrieve_form()

Previewing a node should still be possible without errors.