fix(popout): guard against mid-frame unbind when closing from the header

HellionChat/Ui/Windows/ChannelPopoutWindow.cs

@@ -82,6 +82,12 @@ internal sealed class ChannelPopoutWindow : Window
 
         DrawHeader(Bound);
 
+        // The header close button can unbind us mid-frame (CloseRequested ->
+        // pool.TryClose -> Unbind nulls Bound). Re-check before the body so we
+        // never hand a null tab to MessageList/InputBar in this same Draw call.
+        if (Bound is null)
+            return;
+
         var inputHeight = InputBar.Height;
         using (
             var body = ImRaii.Child(