gistfile1.txt

class PurchaseDialog extends StatefulWidget with NavigationStates {

  @override
  _PurchaseDialogState createState() => _PurchaseDialogState();

  PurchaseDialog();
}

class _PurchaseDialogState extends State<PurchaseDialog> {

  _PurchaseDialogState();

  String title = "Pro Version";
  String description = "Full App Access";
  String purchaseButtonText = "Loading...";
  bool isLoading = true;

// inapp
  StreamSubscription _purchaseUpdatedSubscription;
  StreamSubscription _purchaseErrorSubscription;
  StreamSubscription _conectionSubscription;
  final List<String> _productLists = Platform.isAndroid
      ? [
//    'android.test.purchased',
//    'android.test.canceled',
    'example_pro_inapp_android',
  ]
      : ['example_pro_inapp'];
//

  String _platformVersion = 'Unknown';
  List<IAPItem> _items = [];
  List<PurchasedItem> _purchases = [];

  static const double padding = 20.0;
  final primaryColor = const Color(0xFF75A2EA);
  final grayColor = const Color(0xFF939393);


  void _requestPurchase(IAPItem item) {
    FlutterInappPurchase.instance.requestPurchase(item.productId);
  }

  Future _getProduct() async {
    print("_getProduct");
    List<IAPItem> items;

    try {
      items = await FlutterInappPurchase.instance.getProducts(_productLists);
      print('flutterInappPurchase.instance.getProducts');
    } catch (err) {
      print('flutterInappPurchase.instance.getProducts error: $err');


      Navigator.of(context).pop();
      showDialog(
        context: context,
        builder: (BuildContext context) {
          // return object of type Dialog
          return AlertDialog(
            title: new Text("Failure",
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 33,
                fontWeight: FontWeight.w800,
                color: Color(0xFFF44336 ),
              ),),
            content: new Text("Error. Possibly no internet. Please turn on your internet.",
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 20,
                fontWeight: FontWeight.w400,
              ),),
            actions: <Widget>[
              // usually buttons at the bottom of the dialog
              new FlatButton(
                child: new Text("Close",
                  style: TextStyle(
                    fontSize: 20,
                    fontWeight: FontWeight.w400,
                  ),),
                onPressed: () {
                  Navigator.of(context).pop();
                },
              ),
            ],
          );
        },
      );

      return;

    }



    for (var item in items) {
      print('${item.toString()}');
      this._items.add(item);
    }

    print("items.length");
    print(items.length);
    if (items.length > 0)
      {
        if (items[items.length-1].title == "")
          {
            print("empty title");
          }



        title = items[items.length-1].title;
        description = items[items.length-1].description + "\n" + items[items.length-1].localizedPrice;
        purchaseButtonText = "Purchase";
        isLoading = false;
      }


    setState(() {
      this._items = items;
      this._purchases = [];
    });
  }

  Future _getPurchases() async {
    List<PurchasedItem> items =
    await FlutterInappPurchase.instance.getAvailablePurchases();
    for (var item in items) {
      print('${item.toString()}');
      this._purchases.add(item);
    }

    setState(() {
      this._items = [];
      this._purchases = items;
    });
  }

  Future _getPurchaseHistory() async {
    print("_getPurchaseHistory");

    List<PurchasedItem> items = await FlutterInappPurchase.instance.getPurchaseHistory();

    print(items.length);
    for (var item in items) {
      print('${item.toString()}');
      this._purchases.add(item);
    }

    setState(() {
      this._items = [];
      this._purchases = items;
    });
  }

  // Platform messages are asynchronous, so we initialize in an async method.
  Future<void> initInAppPlatformState() async {
    String platformVersion;
    // Platform messages may fail, so we use a try/catch PlatformException.
    try {
      platformVersion = await FlutterInappPurchase.instance.platformVersion;
    } on PlatformException {
      platformVersion = 'Failed to get platform version.';
    }

    // prepare
    var result = await FlutterInappPurchase.instance.initConnection;
    print('result: $result');

    // If the widget was removed from the tree while the asynchronous platform
    // message was in flight, we want to discard the reply rather than calling
    // setState to update our non-existent appearance.
    if (!mounted) return;

    setState(() {
      _platformVersion = platformVersion;
    });

    // refresh items for android
    try {
      String msg = await FlutterInappPurchase.instance.consumeAllItems;
      print('consumeAllItems: $msg');
    } catch (err) {
      print('consumeAllItems error: $err');
    }

    _purchaseUpdatedSubscription = FlutterInappPurchase.purchaseUpdated.listen((productItem) async {
      print('purchase-updated: $productItem');
      bool result = await savePurchaseStatusPreference(true);
      print(isPurchasedObject.isPurchased);

      Navigator.of(context).pop();

    });

    _purchaseErrorSubscription = FlutterInappPurchase.purchaseError.listen((purchaseError) {
      print('purchase-error: $purchaseError');

      Navigator.of(context).pop();
      showDialog(
        context: context,
        builder: (BuildContext context) {
          // return object of type Dialog
          return AlertDialog(
            title: new Text("Failure",
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 33,
                fontWeight: FontWeight.w800,
                color: Color(0xFFF44336 ),
              ),),
            content: new Text("Error Purchase",
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 20,
                fontWeight: FontWeight.w400,
              ),),
            actions: <Widget>[
              // usually buttons at the bottom of the dialog
              new FlatButton(
                child: new Text("Close",
                  style: TextStyle(
                    fontSize: 20,
                    fontWeight: FontWeight.w400,
                  ),),
                onPressed: () {
                  Navigator.of(context).pop();
                },
              ),
            ],
          );
        },
      );



    });

    _getProduct();
  }


  @override
  void initState(){
    super.initState();
    initInAppPlatformState();
  }


  @override
  void dispose(){
    super.dispose();
  }

  void purchaseOnPressedAction() async
  {
    print("purchaseOnPressedAction");

    if (_items.length > 0)
    {
      print(_items[_items.length-1].productId);
      _requestPurchase(_items[_items.length-1]);
    }
    else
    {
      print("error nothing to purchase");
      Navigator.pop(context);

      showDialog(
        context: context,
        builder: (BuildContext context) {
          // return object of type Dialog
          return AlertDialog(
            title: new Text("Failure",
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 33,
                fontWeight: FontWeight.w800,
                color: Color(0xFFF44336 ),
              ),),
            content: new Text("Error - Nothing To Purchase",
              textAlign: TextAlign.center,
              style: TextStyle(
                fontSize: 20,
                fontWeight: FontWeight.w400,
              ),),
            actions: <Widget>[
              // usually buttons at the bottom of the dialog
              new FlatButton(
                child: new Text("Close",
                  style: TextStyle(
                    fontSize: 20,
                    fontWeight: FontWeight.w400,
                  ),),
                onPressed: () {
                  Navigator.of(context).pop();
                },
              ),
            ],
          );
        },
      );

    }



  }


  @override
  Widget build(BuildContext context) {
    return Dialog(
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(padding),
      ),
      child: Stack(
        children: <Widget>[
          Container(
            padding: EdgeInsets.all(padding),
            decoration: BoxDecoration(
                color: Colors.white,
                shape: BoxShape.rectangle,
                borderRadius: BorderRadius.circular(padding),
                boxShadow: [
                  BoxShadow(
                    color: Colors.black,
                    blurRadius: 10.0,
                    offset: const Offset(0.0, 10.0),
                  ),
                ]),
            child: Column(
              mainAxisSize: MainAxisSize.min,
              children: <Widget>[
                SizedBox(height: 24.0),
                AutoSizeText(
                  title,
                  maxLines: 2,
                  textAlign: TextAlign.center,
                  style: TextStyle(
                    color: primaryColor,
                    fontSize: 25.0,
                  ),
                ),
                SizedBox(height: 24.0),
                AutoSizeText(
                  description,
                  maxLines: 4,
                  textAlign: TextAlign.center,
                  style: TextStyle(
                    color: grayColor,
                    fontSize: 18.0,
                  ),
                ),
                SizedBox(height: 24.0),
                RaisedButton(
                  color: primaryColor,
                  shape: RoundedRectangleBorder(
                      borderRadius: BorderRadius.circular(30.0)),
                  child: Padding(
                    padding: const EdgeInsets.fromLTRB(20, 10, 20, 10),
                    child: AutoSizeText(
                      purchaseButtonText,
                      maxLines: 1,
                      style: TextStyle(
                        fontSize: 18,
                        fontWeight: FontWeight.w400,
                        color: Colors.white,
                      ),
                    ),
                  ),
                  onPressed: (isLoading) ? null : () => purchaseOnPressedAction(),
                ),
                SizedBox(height: 10.0),
                showSecondaryButton(context),
              ],
            ),
          )
        ],
      ),
    );
  }

  showSecondaryButton(BuildContext context) {
      return FlatButton(
        child: AutoSizeText(
          "Cancel",
          maxLines: 1,
          style: TextStyle(
            fontSize: 18,
            color: primaryColor,
            fontWeight: FontWeight.w400,
          ),
        ),
        onPressed: () {
          print("secondary button pressed");
          Navigator.of(context).pop();
 
        },
      );
  }
}

notice in the last part:

showSecondaryButton(BuildContext context) {
....
Navigator.of(context).pop(); <--- this will cause the error below. infact, any pop in the code will cause the same error

[ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'findAncestorStateOfType' was called on null.

PurchaseDialog above is called in this way:

    showDialog(
      context: context,
      builder: (BuildContext context) => PurchaseDialog(),
    ).then((value) {
      setState(() {
      });
    });