fix: complete does not destroy engine on close (#21943)

* fix: complete does not destroy engine on close

* reset flutterApi on cleanup

---------

Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>

mobile/ios/Runner/Background/BackgroundWorker.swift

@@ -133,7 +133,6 @@ class BackgroundWorker: BackgroundWorkerBgHostApi {
       return
     }
 
-    isComplete = true
     flutterApi?.cancel { result in
       self.complete(success: false)
     }
@@ -174,6 +173,7 @@ class BackgroundWorker: BackgroundWorkerBgHostApi {
     
     isComplete = true
     engine.destroyContext()
+    flutterApi = nil
     completionHandler(success)
   }
 }