Use ref for CSSTransition

This squashes the warning about deprecated findDOMNode usage

src/components/contact-form/ContactForm.tsx

@@ -48,6 +48,10 @@ export const ContactForm: React.FC<ContactFormProps> = ({
     setModalContent(JSON.stringify(formData));
   };
 
+  // For use in the CSSTransition, see
+  // https://github.com/reactjs/react-transition-group/issues/668#issuecomment-695162879
+  const submitRef = React.useRef<any>(null);
+
   return (
     <form onSubmit={submit} className={`${styles.ContactForm}`} aria-label={t("Contact form")}>
       <Fieldset legend={t('contact.details')}>
@@ -106,27 +110,29 @@ export const ContactForm: React.FC<ContactFormProps> = ({
             key={urgentIntent ? 'urgent' : 'normal'}
             in={urgentIntent}
             classNames={{ ...panelTransition }}
+            nodeRef={submitRef}
             unmountOnExit={true}
             mountOnEnter={true}
-            addEndListener={(node: HTMLElement, done: EventListener) => {
+            addEndListener={(done: () => void) => {
               // use the css transitionend event to mark the finish of a transition
-              // istanbul ignore next
-              node.addEventListener('transitionend', done, false);
+              submitRef.current?.addEventListener('transitionend', done, false);
             }}
           >
-            {urgentIntent ? (
-              <UrgentRequest sendUrgent={() => sendRequest(true)} />
-            ) : (
-              <button
-                className="good"
-                onClick={(event) => {
-                  event.preventDefault();
-                  sendRequest();
-                }}
-              >
-                {t('submit')}
-              </button>
-            )}
+            <div ref={submitRef}>
+              {urgentIntent ? (
+                <UrgentRequest sendUrgent={() => sendRequest(true)} />
+              ) : (
+                <button
+                  className="good"
+                  onClick={(event) => {
+                    event.preventDefault();
+                    sendRequest();
+                  }}
+                >
+                  {t('submit')}
+                </button>
+              )}
+            </div>
           </CSSTransition>
         </SwitchTransition>
       </div>