Response Header Says null return

postRequest - response Response{protocol=h2, code=200, message=, url=}

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.contains(java.lang.CharSequence)" because the return value of "okhttp3.Response.header(String)" is null

Please let me know what is the issue
Input values are

orderParams.quantity = 1;
orderParams.orderType = Constants.ORDER_TYPE_LIMIT;
orderParams.tradingsymbol = "ASHOKLEY";
orderParams.product = Constants.PRODUCT_CNC; = Constants.EXCHANGE_NSE;
orderParams.transactionType = Constants.TRANSACTION_TYPE_BUY;
//orderParams.validity = Constants.VALIDITY_DAY;
orderParams.validity = Constants.VALIDITY_TTL;
orderParams.price = 122.2;
orderParams.triggerPrice = 120.0;
orderParams.tag = "myTag";
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