Fix crash according to TerminalStudio#68

Author: luxi78

lib/frontend/terminal_view.dart

@@ -209,7 +209,8 @@ class _TerminalViewState extends State<TerminalView> {
           // with widgets such as Scrollbar.
           return NotificationListener<ScrollNotification>(
             onNotification: (notification) {
-              onScroll(notification.metrics.pixels);
+              //onScroll(notification.metrics.pixels);
+              new Future.microtask(() => onScroll(notification.metrics.pixels));
               return false;
             },
             child: ScrollConfiguration(

pubspec.yaml

@@ -4,7 +4,7 @@ version: 2.6.0
 homepage: https://github.com/TerminalStudio/xterm.dart
 
 environment:
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.16.1 <3.0.0"
   flutter: ">=2.0.0"
 
 dependencies: