Closing the game while the workbench is open causes a segmentation fault

[IntegratedQuantum]

This was likely introduced by #943 which moved the disconned before the gui closing actions.

Segmentation fault at address 0x73872b5461a8
Disconnected
/home/mint/Cubyz/src/network.zig:1182:25: 0x159cebf in sendCommand (Cubyzig)
   std.debug.assert(conn.user == null);
                        ^
/home/mint/Cubyz/src/Inventory.zig:58:48: 0x14ed3b0 in executeCommand (Cubyzig)
   main.network.Protocols.inventory.sendCommand(main.game.world.?.conn, cmd.payload, data);
                                               ^
/home/mint/Cubyz/src/Inventory.zig:1794:32: 0x14a28dc in depositOrDrop (Cubyzig)
 Sync.ClientSide.executeCommand(.{.depositOrDrop = .{.dest = dest, .source = source}});
                               ^
/home/mint/Cubyz/src/gui/windows/workbench.zig:100:42: 0x1438f87 in onClose (Cubyzig)
 main.game.Player.inventory.depositOrDrop(inv);
                                         ^
/home/mint/Cubyz/src/gui/gui.zig:166:19: 0x141bb63 in deinit (Cubyzig)
  window.onCloseFn();
                  ^
/home/mint/Cubyz/src/game.zig:618:18: 0x141b95d in deinit (Cubyzig)
  main.gui.deinit();
                 ^
/home/mint/Cubyz/src/main.zig:701:15: 0x140d45d in main (Cubyzig)
  world.deinit();
              ^
/home/mint/Cubyz/compiler/zig/lib/std/start.zig:608:22: 0x140c0b9 in main (Cubyzig)
            root.main();
                     ^