Fix unavailable Play subscription handling

Platforms/Android/AndroidPlayBillingService.cs

@@ -92,12 +92,20 @@ public sealed class AndroidPlayBillingService : Java.Lang.Object, IPlayBillingSe
     {
         try
         {
+            if (string.IsNullOrWhiteSpace(basePlanId))
+            {
+                return PlayBillingResult.Failed("Piano LME Pro non valido.");
+            }
+
             var details = await GetProductDetailsAsync(cancellationToken);
             var offer = details?.GetSubscriptionOfferDetails()?
-                .FirstOrDefault(item => item.BasePlanId == basePlanId && string.IsNullOrEmpty(item.OfferId));
+                .FirstOrDefault(item => item.BasePlanId == basePlanId
+                    && string.IsNullOrEmpty(item.OfferId)
+                    && !string.IsNullOrWhiteSpace(item.OfferToken));
             if (details is null || offer is null)
             {
-                return PlayBillingResult.Failed("Piano LME Pro non disponibile. Verifica prodotto e base plan nella Play Console.");
+                return PlayBillingResult.Failed(
+                    $"Piano {basePlanId} non disponibile su Google Play. Verifica che l'abbonamento lme_pro e il piano siano attivi per il tuo Paese.");
             }
 
             var productParams = BillingFlowParams.ProductDetailsParams.NewBuilder()
@@ -113,7 +121,8 @@ public sealed class AndroidPlayBillingService : Java.Lang.Object, IPlayBillingSe
             var completion = new TaskCompletionSource<PlayBillingResult>(TaskCreationOptions.RunContinuationsAsynchronously);
             _purchaseCompletion = completion;
             using var registration = cancellationToken.Register(() => completion.TrySetCanceled(cancellationToken));
-            var launchResult = _client.LaunchBillingFlow(activity, flow);
+            var launchResult = await MainThread.InvokeOnMainThreadAsync(() =>
+                _client.LaunchBillingFlow(activity, flow));
             if (launchResult.ResponseCode != BillingResponseCode.Ok)
             {
                 _purchaseCompletion = null;
@@ -182,7 +191,7 @@ public sealed class AndroidPlayBillingService : Java.Lang.Object, IPlayBillingSe
             throw new InvalidOperationException(ToMessage(result.Result));
         }
 
-        return result.ProductDetailsList.FirstOrDefault();
+        return result.ProductDetailsList?.FirstOrDefault();
     }
 
     private async Task EnsureConnectedAsync(CancellationToken cancellationToken)