Problem with form theming

Hello!

I read about theming forms at http://api.drupal.org/api/file/developer/topics/forms_api.html/6
Now I tried this for myself inside my own module.
But I get strange results. First, the submit button in the fieldset is above the textfield, although I expected it the other way around (cause I first rendered textfield and then the submit button).
Also I get those five <br>'s below the fieldset, although I rendered it in between.
Second, I get an error message (warning: implode() [function.implode]: Bad arguments. in /var/www/medicalgenomics_drupal/includes/form.inc on line 621.)

What am I doing wrong? Can't find a really good example code.

Here my code:

/* $Id$ */

function blotbase_search_perm() {
	return array('access blotbase_search');
}

function _main() {
	//$content = drupal_get_form('_query_form');
	$content = theme_query_form(_query_form());
	return $content;
}

function _query_form() {
	$form["fulltext_search"] = array(
		'#type' => 'fieldset',
		'#title' => t('Search BlotBase'),
	);
	$form["fulltext_search"]["action"] = array(
		'#type' => 'hidden',
		'#value' => 'query',
	);
	$form["fulltext_search"]["query_string"] = array(
		'#type' => 'textfield',
		'#title' => t('Search for a gene symbol resp. identifier and/or paper title/author'),
		'#default_value' => '',
		'#size' => 60,
		'#maxlength' => 64,
		'#description' => t('baz'),
	);
	$form["fulltext_search"]["submit"] = array(
		'#type' => 'submit',
		'#value' => t('Search'),
	);
	return $form;
}

function theme_query_form($form) {
	$output = '';
	$output .= drupal_render($form["fulltext_search"]);
	$output .= drupal_render($form["fulltext_search"]["query_string"]);
	$output .= '<br><br><br><br><br>';
	$output .= drupal_render($form["fulltext_search"]["submit"]);
	$output .= drupal_render($form);
	return $output;
}

function blotbase_search_menu() {
	$items[] = array(
		'path' => 'databases/blotbase',
		'title' => 'Search BlotBase',
		'callback' => '_main',
		'access' => user_access('access blotbase_search'),
		'type' => MENU_CALLBACK,
	);
	return $items;
}