Add null check for ListDetailsView.OnListPaneWidthChanged#4817

nodew wants to merge 1 commit intoCommunityToolkit:rel/winui/7.1.2CommunityToolkit/WindowsCommunityToolkit:rel/winui/7.1.2from

nodew:fix-winui-list-details-viewnodew/WindowsCommunityToolkit:fix-winui-list-details-viewCopy head branch name to clipboard