Fix theming issues

lib/ui/widgets/tablet_mode/memberships_panel.dart

@@ -171,8 +171,7 @@ class _MembershipsPanelState extends SafeState<MembershipsPanel> {
     try {
       if (!accept &&
           !await showConfirmDialog(
-              context: Scaffold.of(context)
-                  .context, // Without this the text appears in white
+              context: context,
               message:
                   tr("Do you really want to reject this friendship request?")))
         return;

lib/utils/ui_utils.dart

@@ -146,8 +146,12 @@ Future<bool> showConfirmDialog({
 }) async {
   if (title == null) title = tr("Confirm operation");
 
+  // Avoid potential theme issues
+  var ctx = Scaffold.of(context).context;
+  ctx = ctx!= null ? ctx : context;
+
   final result = await showDialog<bool>(
-      context: context,
+      context: ctx,
       builder: (c) => AlertDialog(
             title: Text(title),
             content: Text(message),