fix Windows build

src/widgets/Ruler.cpp

@@ -1956,6 +1956,9 @@ AdornedRulerPanel::AdornedRulerPanel(AudacityProject* parent,
 , mProject(parent)
 , mViewInfo(viewinfo)
 {
+   for (auto &button : mButtons)
+      button = nullptr;
+
    ReCreateButtons();
 
    SetLabel( _("Timeline") );

src/widgets/Ruler.h

@@ -446,7 +446,7 @@ private:
 
    friend QuickPlayRulerOverlay;
 
-   wxWindow *mButtons[1] { {} };
+   wxWindow *mButtons[1];
    bool mNeedButtonUpdate { true };
 };