PROGRESSION (263656@main): Three fast/scrolling/mac/scrollbars are a consistent failure or timeout

https://bugs.webkit.org/show_bug.cgi?id=258641
rdar://111424377

Reviewed by Tim Nguyen.

Add check for scroller before querying the type. This regressed after
https://commits.webkit.org/263656@main which added the type check for the select element
tests, which need to use the old scrollbar testing infrastructure.

* LayoutTests/resources/ui-helper.js:

Canonical link: https://commits.webkit.org/265728@main

Author: nmoucht

LayoutTests/platform/mac-wk2/TestExpectations

@@ -1106,7 +1106,6 @@ webkit.org/b/188924 fast/mediastream/device-change-event-2.html [ Pass Timeout ]
 
 webkit.org/b/212721 svg/custom/textPath-change-id.svg [ Pass ImageOnlyFailure ]
 
-webkit.org/b/212042 [ Debug ] fast/scrolling/mac/scrollbars/overlay-scrollbar-reveal.html [ Pass Timeout ]
 webkit.org/b/212042 [ Debug ] fast/scrolling/mac/scrollbars/select-overlay-scrollbar-reveal.html [ Pass Timeout ]
 
 webkit.org/b/213461 fast/mediastream/mediastreamtrack-video-frameRate-clone-increasing.html [ Pass Failure ]
@@ -1211,14 +1210,10 @@ webkit.org/b/230113 [ Monterey+ Debug arm64 ] svg/animations/animate-elem-14-t-d
 
 webkit.org/b/230696 imported/w3c/web-platform-tests/webrtc/RTCDataChannel-close.html [ Pass Failure ]
 
-webkit.org/b/215611 [ Debug ] fast/scrolling/mac/scrollbars/overflow-overlay-scrollbar-hovered.html [ Pass Timeout ]
-
 # rdar://65269589 ([ macOS and iOS ] media/vp9.html is failing consistently.)
 [ Monterey+ ] media/vp9.html [ Pass ]
 [ Monterey+ ] webrtc/vp9-vtb.html [ Pass ]
 
-webkit.org/b/215700 fast/scrolling/mac/scrollbars/overlay-scrollbar-hovered.html [ Pass Failure Timeout ]
-
 webkit.org/b/214997 [ Release ] http/tests/workers/service/basic-install-event-waitUntil-reject.html [ Pass Timeout ]
 
 webkit.org/b/215809 [ Debug ] media/W3C/video/events/event_order_canplay_canplaythrough.html [ Pass Timeout ]

LayoutTests/resources/ui-helper.js

@@ -855,7 +855,7 @@ window.UIHelper = class UIHelper {
         if (!this.isWebKit2() || this.isIOSFamily())
             return Promise.resolve();
 
-        if (internals.isUsingUISideCompositing() && scroller.nodeName != "SELECT") {
+        if (internals.isUsingUISideCompositing() && (!scroller || scroller.nodeName != "SELECT")) {
             return new Promise(resolve => {
                 testRunner.runUIScript(`(function() {
                     uiController.doAfterNextStablePresentationUpdate(function() {