Index: wp-admin/network/themes.php
===================================================================
--- wp-admin/network/themes.php	(revision 17229)
+++ wp-admin/network/themes.php	(working copy)
@@ -95,6 +95,10 @@
 			$main_theme = get_current_theme();
 			$files_to_delete = $theme_info = array();
 			foreach ( $themes as $key => $theme ) {
+				if ( ! file_exists( WP_CONTENT_DIR . '/themes/' . $theme . '/style.css' ) ) {
+					unset( $themes[$key] );
+					continue;
+				}
 				$data = get_theme_data( WP_CONTENT_DIR . '/themes/' . $theme . '/style.css' );
 				if ( $data['Name'] == $main_theme ) {
 					unset( $themes[$key] );