fix for upcoming use_build_context_synchronously (#782)

diff --git a/lib/pages/demo.dart b/lib/pages/demo.dart
index 6a3255c..aff7dd4 100644
--- a/lib/pages/demo.dart
+++ b/lib/pages/demo.dart
@@ -194,7 +194,7 @@
 
     if (await canLaunchUrlString(url)) {
       await launchUrlString(url);
-    } else {
+    } else if (mounted) {
       await showDialog<void>(
         context: context,
         builder: (context) {