[popover] Adjust showPopover

https://bugs.webkit.org/show_bug.cgi?id=258800

Reviewed by Tim Nguyen.

Adjust to spec change:
whatwg/html#9439

There should be no behavior change that can be tested.

* Source/WebCore/html/HTMLElement.cpp:
(WebCore::HTMLElement::showPopover):

Canonical link: https://commits.webkit.org/265734@main

Source/WebCore/html/HTMLElement.cpp

@@ -1349,15 +1349,15 @@ void HTMLElement::queuePopoverToggleEventTask(PopoverVisibilityState oldState, P
 
 ExceptionOr<void> HTMLElement::showPopover(const HTMLFormControlElement* invoker)
 {
-    if (popoverData())
-        popoverData()->setInvoker(invoker);
-
     auto check = checkPopoverValidity(*this, PopoverVisibilityState::Hidden);
     if (check.hasException())
         return check.releaseException();
     if (!check.returnValue())
         return { };
 
+    if (popoverData())
+        popoverData()->setInvoker(invoker);
+
     ASSERT(!isInTopLayer());
 
     PopoverData::ScopedStartShowingOrHiding showOrHidingPopoverScope(*this);