ContextWindow: Dispose context after creating new one

Elwig/Windows/ContextWindow.cs

@@ -46,10 +46,11 @@ namespace Elwig.Windows {
 
         protected async Task RenewContext() {
             if (!_renewPending) return;
-            Context.Dispose();
+            var old = Context;
             Context = new();
             await OnRenewContext();
             _renewPending = false;
+            old.Dispose();
         }
 
         abstract protected Task OnRenewContext();