fix: issue in fallback config

Author: alexander-heimbuch

src/plugin/includes/class-podlove-web-player-options.php

@@ -156,7 +156,7 @@ private function fallbackConfig($config)
                 'sharePlaytime' => (is_bool($share['sharePlaytime']) ? $share['sharePlaytime'] : true)
             ),
             'related-episodes' => array(
-                'source' => (is_string($relatedEpisodes['source']) ? $relatedEpisodes['source'] : 'disabled'),
+                'source' => (isset($relatedEpisodes['source']) ? $relatedEpisodes['source'] : 'disabled'),
                 'value' => $relatedEpisodes['value'] ?? null,
             ),
         );