umebius様返信ありがとうございました。
そのあたりの設定は一度見直しましたので問題ないかと思いましたが、ご指摘を受けよくよく調べてみるとご指摘の「お届け日調整プラグイン」が原因でした。

リリースノートの以下の修正-------------
発送受付可能時間の設定の拡張(ver.1.1.0)（2020/05/25）
即日発送受付可能時間の機能を即日発送商品だけではなく、全商品を対象に出来る設定を追加いたしました。
------------------------------
この修正をアップデートした際に
私の環境では何も触らずに運営すると今回のエラーになるようです。

新しく追加された
全商品を対象とする（即日発送商品以外も対象） との項目のチェックボックスにチェックを入れたらエラーが解消されました。
その直後チェックを外してもエラーは発生しませんでした。
恐らくアップデートした後にプラグインの設定画面で設定の変更の有無に関わらず設定を登録する動作が必要であったのかと思います。

推測を含みますが解決したと思います。ご協力いただきありがとうございました。

他のプラグインやカスタマイズと競合しているかもしれないですが、
DeliveryDateというプラグインのコードでエラーが発生しているようですね。
お心当たりありますでしょうか？

app/Plugin/DeliveryDate/Service/ShoppingService.php,"line":64

▼テンプレート
[EC-CUBE] 3.0.18
[レンタルサーバ]
[OS]
[PHP] 7.3.15
[データベース] PostgreSQL 8.4.20
[WEBサーバ]


[現象] 再現手順や、どのような現象が発生しているかなど、具体的に
カートからレジに進めずエラーが出ます。

先日まで問題なく稼働しておりましたがエラーが発生するようになってしまいました。
会員はカートの画面の次で、非会員は住所等を登録した画面の次の遷移でエラーとなりました。

心当たりとしては
「商品一括公開非公開プラグイン」をインストールして数百件近い数の商品を一括非公開にしてそのあとすぐにそのプラグインをアンインストールしました。
しかし問題を認識するまでに時間がかかりそれが原因か確証が持てません。

よろしくお願いします。

エラーログを以下にペーストします
[2020-07-12 10:44:31,379128] front.ERROR [43deceaa] [c8472592] [2] [EccubeExceptionHandler:handle:44] - Call to a member function getValue() on null [{"namespace":"","short_class":"","class":"","type":"","function":"","file":"/XXX/ec/app/Plugin/DeliveryDate/Service/ShoppingService.php","line":64,"args":[]},{"namespace":"Plugin\\DeliveryDate\\Service","short_class":"ShoppingService","class":"Plugin\\DeliveryDate\\Service\\ShoppingService","type":"->","function":"getFormDeliveryDates","file":"/XXX/ec/app/Plugin/DeliveryDate/Form/Extension/ShippingItemExtension.php","line":50,"args":[["object","Eccube\\Entity\\Shipping"],["object","Eccube\\Entity\\Delivery"]]},{"namespace":"Plugin\\DeliveryDate\\Form\\Extension","short_class":"ShippingItemExtension","class":"Plugin\\DeliveryDate\\Form\\Extension\\ShippingItemExtension","type":"->","function":"Plugin\\DeliveryDate\\Form\\Extension\\{closure}","file":null,"line":null,"args":[["object","Symfony\\Component\\Form\\FormEvent"],["string","form.pre_set_data"],["object","Symfony\\Component\\EventDispatcher\\EventDispatcher"]]},{"namespace":"","short_class":"","class":"","type":"","function":"call_user_func","file":"/XXX/ec/vendor/symfony/event-dispatcher/EventDispatcher.php","line":161,"args":[["object","Closure"],["object","Symfony\\Component\\Form\\FormEvent"],["string","form.pre_set_data"],["object","Symfony\\Component\\EventDispatcher\\EventDispatcher"]]},{"namespace":"Symfony\\Component\\EventDispatcher","short_class":"EventDispatcher","class":"Symfony\\Component\\EventDispatcher\\EventDispatcher","type":"->","function":"doDispatch","file":"/XXX/ec/vendor/symfony/event-dispatcher/EventDispatcher.php","line":46,"args":[["array",[["object","Closure"],["object","Closure"],["object","Closure"]]],["string","form.pre_set_data"],["object","Symfony\\Component\\Form\\FormEvent"]]},{"namespace":"Symfony\\Component\\EventDispatcher","short_class":"EventDispatcher","class":"Symfony\\Component\\EventDispatcher\\EventDispatcher","type":"->","function":"dispatch","file":"/XXX/ec/vendor/symfony/event-dispatcher/ImmutableEventDispatcher.php","line":33,"args":[["string","form.pre_set_data"],["object","Symfony\\Component\\Form\\FormEvent"]]},{"namespace":"Symfony\\Component\\EventDispatcher","short_class":"ImmutableEventDispatcher","class":"Symfony\\Component\\EventDispatcher\\ImmutableEventDispatcher","type":"->","function":"dispatch","file":"/XXX/ec/vendor/symfony/form/Form.php","line":340,"args":[["string","form.pre_set_data"],["object","Symfony\\Component\\Form\\FormEvent"]]},{"namespace":"Symfony\\Component\\Form","short_class":"Form","class":"Symfony\\Component\\Form\\Form","type":"->","function":"setData","file":"/XXX/ec/vendor/symfony/form/Extension/Core/DataMapper/PropertyPathMapper.php","line":49,"args":[["object","Eccube\\Entity\\Shipping"]]},{"namespace":"Symfony\\Component\\Form\\Extension\\Core\\DataMapper","short_class":"PropertyPathMapper","class":"Symfony\\Component\\Form\\Extension\\Core\\DataMapper\\PropertyPathMapper","type":"->","function":"mapDataToForms","file":"/XXX/ec/vendor/symfony/form/Form.php","line":384,"args":[["object","Doctrine\\ORM\\PersistentCollection"],["object","RecursiveIteratorIterator"]]},{"namespace":"Symfony\\Component\\Form","short_class":"Form","class":"Symfony\\Component\\Form\\Form","type":"->","function":"setData","file":"/XXX/ec/vendor/symfony/form/Extension/Core/DataMapper/PropertyPathMapper.php","line":51,"args":[["object","Doctrine\\ORM\\PersistentCollection"]]},{"namespace":"Symfony\\Component\\Form\\Extension\\Core\\DataMapper","short_class":"PropertyPathMapper","class":"Symfony\\Component\\Form\\Extension\\Core\\DataMapper\\PropertyPathMapper","type":"->","function":"mapDataToForms","file":"/XXX/ec/vendor/symfony/form/Form.php","line":384,"args":[["null",null],["object","RecursiveIteratorIterator"]]},{"namespace":"Symfony\\Component\\Form","short_class":"Form","class":"Symfony\\Component\\Form\\Form","type":"->","function":"setData","file":"/XXX/ec/vendor/symfony/form/Form.php","line":488,"args":[["null",null]]},{"namespace":"Symfony\\Component\\Form","short_class":"Form","class":"Symfony\\Component\\Form\\Form","type":"->","function":"initialize","file":"/XXX/ec/vendor/symfony/form/FormBuilder.php","line":226,"args":[]},{"namespace":"Symfony\\Component\\Form","short_class":"FormBuilder","class":"Symfony\\Component\\Form\\FormBuilder","type":"->","function":"getForm","file":"/XXX/ec/src/Eccube/Controller/ShoppingController.php","line":138,"args":[]},{"namespace":"Eccube\\Controller","short_class":"ShoppingController","class":"Eccube\\Controller\\ShoppingController","type":"->","function":"index","file":"/XXX/ec/vendor/symfony/http-kernel/HttpKernel.php","line":135,"args":[["object","Eccube\\Application"],["object","Symfony\\Component\\HttpFoundation\\Request"]]},{"namespace":"Symfony\\Component\\HttpKernel","short_class":"HttpKernel","class":"Symfony\\Component\\HttpKernel\\HttpKernel","type":"->","function":"handleRaw","file":"/XXX/ec/vendor/symfony/http-kernel/HttpKernel.php","line":57,"args":[["object","Symfony\\Component\\HttpFoundation\\Request"],["string","1"]]},{"namespace":"Symfony\\Component\\HttpKernel","short_class":"HttpKernel","class":"Symfony\\Component\\HttpKernel\\HttpKernel","type":"->","function":"handle","file":"/XXX/ec/vendor/silex/silex/src/Silex/Application.php","line":586,"args":[["object","Symfony\\Component\\HttpFoundation\\Request"],["string","1"],["boolean",true]]},{"namespace":"Silex","short_class":"Application","class":"Silex\\Application","type":"->","function":"handle","file":"/XXX/ec/vendor/silex/silex/src/Silex/Application.php","line":563,"args":[["object","Symfony\\Component\\HttpFoundation\\Request"]]},{"namespace":"Silex","short_class":"Application","class":"Silex\\Application","type":"->","function":"run","file":"/XXX/ec/html/index.php","line":57,"args":[]}] [GET, /ec/html/shopping, XX.XXx.XXX.XX, https://www.XXXXXXX/ec/html/cart, Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko]
[2020-07-12 10:44:31,383389] front.INFO [43deceaa] [c8472592] [2] [LogListener:onKernelTerminate:95] - PROCESS END ["shopping"] [GET, /ec/html/shopping, XX.XXX.XXX.X, https://www.XXXXXXX/ec/html/cart, Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko]