Steps to reproduce the issue

The model /administrator/components/com_modules/models/module.php of the component com_modules can load the form data from the session - method loadFormData contains at the start:

$data = JFactory::getApplication()->getUserState('com_modules.edit.module.data', array());

but controllers don't contain methods to set the session variable 'com_modules.edit.module.data'.

I suggest add to controller /administrator/components/com_modules/controllers/module.php the method setData:

	public function setData()
	{
		JSession::checkToken() or jexit(JText::_('JINVALID_TOKEN'));

		$data = $this->input->post->get('jform', array(), 'array');
		JFactory::getApplication()->setUserState('com_modules.edit.module.data', $data);
		$this->setRedirect(JRoute::_('index.php?option=' . $this->option . '&view=module' . $this->getRedirectToItemAppend($this->input->get('id')), false));
	}

Then we can create custom module fields with relations i.e. field 'extension' - field 'category'.
Similar behavior has a field 'Menu item type'.

Expected result

Actual result

System information (as much as possible)

Additional comments

Joomla 3.6.2