Looking up a deactivated widget's ancestor is unsafe

Declare a global variable

    final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();

then register the key on your widget build's scaffold eg

    @override    Widget build(BuildContext context) {     return Scaffold(       key: _scaffoldKey,       ...

then on the dialog

show(BuildContext context){var dialog = Dialog(  child: Container(    margin: EdgeInsets.all(8.0),    child: Form(      child: Column(        mainAxisSize: MainAxisSize.min,        children: <Widget>[          TextFormField(            decoration: InputDecoration(                labelText: "Insira o número de telefone",                border: OutlineInputBorder(                    borderRadius: BorderRadius.all(Radius.circular(2.0)))),          ),          Row(            mainAxisAlignment: MainAxisAlignment.end,            children: <Widget>[              FlatButton(                  onPressed: () {                    Navigator.of(context).pop();                  },                  child: Text("Cancelar")),              FlatButton(                  onPressed: () {                    Navigator.of(context).pop();                  },                  child: Text("Aceitar"))            ],          )        ],      ),    ),  ),);

Pass that scaffold context to the showDialog method

showDialog(context: _scaffoldKey.currentContext ,builder: (context){  return dialog; });}

Try This

Give different context name for dialog

 showDialog(context: context,builder: (dialogContex){              return Dialog(                child: Container(                  margin: EdgeInsets.all(8.0),                  child: Form(                    child: Column(                      mainAxisSize: MainAxisSize.min,                      children: <Widget>[                        TextFormField(                          decoration: InputDecoration(                              labelText: "Insira o número de telefone",                              border: OutlineInputBorder(                                  borderRadius: BorderRadius.all(Radius.circular(2.0)))),                        ),                        Row(                          mainAxisAlignment: MainAxisAlignment.end,                          children: <Widget>[                            FlatButton(                                onPressed: () {                                  Navigator.of(dialogContex).pop();                                },                                child: Text("Cancelar")),                            FlatButton(                                onPressed: () {                                  Navigator.of(context).pop();                                },                                child: Text("Aceitar"))                          ],                        )                      ],                    ),                  ),                ),              );            });

I got the same error when attempting to open a dialog and I found a solution here: github flutter issues. Specifically, I followed the poster's recommendation, which was to create a GlobalKey and associate it with the Scaffold widget, and use the context from that key when creating the dialog. In my case, I have a globally accessible object which holds the GlobalKey:

MyGlobals myGlobals = MyGlobals();class MyGlobals {  GlobalKey _scaffoldKey;  MyGlobals() {    _scaffoldKey = GlobalKey();  }  GlobalKey get scaffoldKey => _scaffoldKey;}

In the Scaffold widget constructor call:

Scaffold(  appBar: ...,  body: ...,  drawer: ...,  key: myGlobals.scaffoldKey,)

And in the showDialog call:

showDialog<String>(  barrierDismissible: ...,  builder: ...,  context: myGlobals.scaffoldKey.currentContext,);