Fix unhandled exception when closing panel with connections

Co-authored-by: Kvarkas <3611964+Kvarkas@users.noreply.github.com>

mRemoteNG/UI/Window/ConnectionWindow.cs

@@ -776,6 +776,7 @@ namespace mRemoteNG.UI.Window
             ProtocolBase protocolBase = sender as ProtocolBase;
             if (!(protocolBase?.InterfaceControl.Parent is ConnectionTab tabPage)) return;
             if (tabPage.Disposing || tabPage.IsDisposed) return;
+            if (IsDisposed || Disposing) return;
             tabPage.protocolClose = true;
             Invoke(new Action(() => tabPage.Close()));
         }