Changed to an exception so it is at least understandable why it's not working.

Author: TiiFuchs

src/Request.php

@@ -323,14 +323,14 @@ public static function setClient(ClientInterface $client): void
      */
     public static function setCustomBotApiUri(string $api_base_uri, string $api_base_download_uri = ''): void
     {
+        if (self::$client) {
+            throw new TelegramException('setCustomBotApiUri() needs to be called before the Telegram object gets instantiated.');
+        }
+
         self::$api_base_uri = $api_base_uri;
         if ($api_base_download_uri !== '') {
             self::$api_base_download_uri = $api_base_download_uri;
         }
-
-        if (self::$telegram) {
-            self::initialize(self::$telegram);
-        }
     }
 
     /**