PopupMenu: Fix issue where PopupMenu would sometimes use the default rather than the parent look and feel

Previously, for the following snippet, the menu's LnF was incorrectly
being forced to the default LnF. The correct behaviour is to display the
menu using LnF v4. The menu doesn't have an explicit LnF set, so it
should use the LnF of its parent component.

    LookAndFeel::setDefaultLookAndFeel (&lookAndFeel_V1);
    setLookAndFeel (&lookAndFeel_V4);
    PopupMenu().showMenuAsync (PopupMenu::Options{}.withParentComponent (this));

modules/juce_gui_basics/menus/juce_PopupMenu.cpp

@@ -333,7 +333,7 @@ struct MenuWindow  : public Component
                 float parentScaleFactor = 1.0f)
         : Component ("menu"),
           parent (parentWindow),
-          options (opts.withParentComponent (findLookAndFeel (menu, parentWindow)->getParentComponentForMenuOptions (opts))),
+          options (opts.withParentComponent (findNonNullLookAndFeel (menu, parentWindow).getParentComponentForMenuOptions (opts))),
           managerOfChosenCommand (manager),
           componentAttachedTo (options.getTargetComponent()),
           dismissOnMouseUp (shouldDismissOnMouseUp),
@@ -1296,13 +1296,16 @@ struct MenuWindow  : public Component
 
     LookAndFeel* findLookAndFeel (const PopupMenu& menu, MenuWindow* parentWindow) const
     {
-        if (parentWindow != nullptr)
-            return &(parentWindow->getLookAndFeel());
+        return parentWindow != nullptr ? &(parentWindow->getLookAndFeel())
+                                       : menu.lookAndFeel.get();
+    }
 
-        if (auto* lnf = menu.lookAndFeel.get())
-            return lnf;
+    LookAndFeel& findNonNullLookAndFeel (const PopupMenu& menu, MenuWindow* parentWindow) const
+    {
+        if (auto* result = findLookAndFeel (menu, parentWindow))
+            return *result;
 
-        return &getLookAndFeel();
+        return getLookAndFeel();
     }
 
     //==============================================================================