title: Requested flutter engine does not exist in Add to app
body: <p>I have flutter into the existing native app (Add to app) with a cached flutter engine.</p>
  <pre><code>override fun onCreate(savedInstanceState: Bundle?) {
          Log.d(TAG, &quot;onCreate&quot;)
          
          
          super.onCreate(savedInstanceState)
  
          setContentView(R.layout.activity_main)
          context = this.application.applicationContext
          if (!FlutterEngineCache.getInstance().contains(
                  FLUTTER_ENGINE
              )
          ) {
              mFlutterEngine = FlutterEngine(context)
              mFlutterEngine.dartExecutor
                  .executeDartEntrypoint(DartExecutor.DartEntrypoint.createDefault())
              FlutterEngineCache.getInstance()
                  .put(FLUTTER_ENGINE, mFlutterEngine)
          }
          //Create Flutter Fragment
          mFragmentManager = supportFragmentManager
          mFlutterFragment = mFragmentManager.findFragmentByTag(FLUTTER_FRAGMENT) as FlutterFragment?
          if (mFlutterFragment == null) {
              mFlutterFragment =
                  FlutterFragment.withCachedEngine(FLUTTER_ENGINE).transparencyMode(FlutterView.TransparencyMode.opaque).build()
              mFragmentManager
                  .beginTransaction()
                  .add(R.id.fragment_container, mFlutterFragment as Fragment, FLUTTER_FRAGMENT)
                  .commit()
          } else {
              mFragmentManager
                  .beginTransaction()
                  .show(mFragmentManager.findFragmentByTag(FLUTTER_FRAGMENT)!!)
                  .commit()
          }
  }
  </code></pre>
  <p>I am getting the following crash in the production app</p>
  <pre><code>  Caused by: java.lang.IllegalStateException: The requested cached FlutterEngine did not exist in the FlutterEngineCache: 'FLUTTER_ENGINE'
          at io.flutter.embedding.android.FlutterActivityAndFragmentDelegate.setupFlutterEngine(FlutterActivityAndFragmentDelegate.java:211)
          at io.flutter.embedding.android.FlutterActivityAndFragmentDelegate.onAttach(FlutterActivityAndFragmentDelegate.java:157)
          at io.flutter.embedding.android.FlutterFragment.onAttach(FlutterFragment.java:597)
          at androidx.fragment.app.Fragment.performAttach(Fragment.java:2574)
          at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:828)
          at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
          at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
          at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2659)
          at androidx.fragment.app.FragmentManagerImpl.dispatchCreate(FragmentManagerImpl.java:2607)
          at androidx.fragment.app.FragmentController.dispatchCreate(FragmentController.java:235)
          at androidx.fragment.app.FragmentActivity.onCreate(FragmentActivity.java:316)
          at androidx.appcompat.app.AppCompatActivity.onCreate(AppCompatActivity.java:106)
          at com.flutter.android.MainActivity.onCreate(MainActivity.kt:33)
   
  </code></pre>