FIX: Leaking a promise (#6)

Signed-off-by: Yoshimasa Niwa <niw@niw.at>

Author: niw

Sources/XCBBuildServiceProxyKit/Hybrid/HybridXCBBuildService.swift

@@ -25,8 +25,8 @@ public final class HybridXCBBuildService<RequestHandler: HybridXCBBuildServiceRe
                     // When the channel for XCBBuildService is closed, such as the process is terminated,
                     // close the channel for Xcode as well, which allows to terminate proxy process.
                     // Xcode then relaunch it when it's needed.
-                    _ = xcbBuildService.channel.closeFuture.flatMap {
-                        channel.close()
+                    xcbBuildService.channel.closeFuture.whenComplete { _ in
+                        channel.close(promise: nil)
                     }
 
                     return channel.pipeline.addHandlers([