The method 'call' was called on null. Receiver: null Tried calling: call(navegacion_bloques) - flutter

Question

give me some of your time please, I have this error "mainMenu().onMenuTap()" and I don't know how to fix it.
The following NoSuchMethodError was thrown while handling a gesture:
The method 'call' was called on null.
Receiver: null
Tried calling: call(navegacion_bloques)
in my StatelessWidget class it worked fine, but I passed everything to a Statefulwidget and I can't use this function.
My code is this:
import 'package:flutter/material.dart';
import 'package:example/bloc/navegacion_bloques/navegacion_bloques.dart';
import 'package:example/ui/pages/productos/categorias.dart';
import 'package:example/ui/pages/productos/productdetails.dart';
class mainMenu extends StatefulWidget with NavigationStates{
final Function onMenuTap;
const mainMenu({Key key, this.onMenuTap}) : super(key: key);
#override
_mainMenu createState() => _mainMenu();
}
class _mainMenu extends State<mainMenu>
{
final categorias= allCategories;
#override
Widget build(BuildContext context) {
return SingleChildScrollView(
scrollDirection: Axis.vertical,
physics: ClampingScrollPhysics(),
child: Container(
decoration: BoxDecoration(
gradient: LinearGradient(
colors: [Colors.deepPurple[300], Colors.deepPurpleAccent]
),
borderRadius:BorderRadius.only(
bottomLeft: Radius.circular(30),
bottomRight: Radius.circular(30),
topLeft: Radius.circular(20),
topRight: Radius.circular(20)),
),
padding: const EdgeInsets.only(left: 16.0, right: 16.0, top: 40),
child: Column(
children: <Widget>[
Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
mainAxisSize: MainAxisSize.max,
children:[
InkWell(child: Icon(Icons.menu,color: Colors.grey[300],),
onTap:(){
mainMenu().onMenuTap();
},),
Text(
"",
style: TextStyle(
color: Colors.white,
fontSize: 22,
fontFamily: 'BalooMedium'),
),
InkWell(child: Icon(Icons.shopping_cart,color: Colors.grey[300],),
onTap:(){
mainMenu().onMenuTap();
},),
],
),
SizedBox(
height: 5,
),
Container(
padding: const EdgeInsets.only(left: 10.0,right: 10.0),
child: Column(
children: <Widget>[
Row(
children:[
Text(
"store",
style: TextStyle(
color: Colors.white,
fontSize: 28,
fontFamily: 'BalooMedium'),
)
],
),
Row(
children:[
Text(
"hi",
style: TextStyle(
color: Colors.white,
fontSize: 15,
fontFamily: 'BalooMedium'),
)
],
),
Row(
children:[
Text(
"There!",
style: TextStyle(
color: Colors.white,
fontSize: 15,
fontFamily: 'BalooMedium'),
)
],
)
],
),
),
SizedBox(
height: 10,
),
Padding(
padding: EdgeInsets.only(left: 10.0,right: 10.0),
child: TextField(
style: TextStyle(
fontFamily: 'BalooMedium',
fontSize: 18.0,
color: Colors.grey[200],
),
decoration: InputDecoration(
prefixIcon: IconButton(icon: Icon(Icons.search,color: Colors.grey[200],),onPressed: null,),
hintText: '¿Qué Necesitas?',
hintStyle:TextStyle(
fontFamily: 'BalooMedium',
fontSize: 18.0,
color: Colors.grey[200],
),
suffixIcon: IconButton(icon: Icon(Icons.filter_list,color: Colors.grey[200],),onPressed: null,),
),
),
),
],
),
),
);
}
buildCategoriasGrid(Categorias categorias){
return GestureDetector(
onTap: () {
Navigator.of(context).push(MaterialPageRoute(
//builder: (context) => productdetails(selectedInstructor: categorias)
));
},
child: Padding(
padding: EdgeInsets.all(5.0),
child: Stack(
children: <Widget>[
Container(
height: 150.0, width: 100.0, color: Colors.transparent),
Positioned(
left: 30.0,
top: 65.0,
child: Container(
height: 30.0,
width: 40.0,
decoration: BoxDecoration(boxShadow: [
BoxShadow(
blurRadius: 7.0,
color: Colors.grey.withOpacity(0.75),
offset: Offset(5, 25),
spreadRadius: 12.0)
]))),
Positioned(
left: 12.0,
top: 15.0,
child: Hero(
tag: categorias.productImg,
child: Container(
height: 110.0,
width: 85.0,
decoration: BoxDecoration(
borderRadius: BorderRadius.circular(7.0),
image: DecorationImage(
image: AssetImage(categorias.productImg),
fit: BoxFit.cover))))),
Positioned(
left: 22.0,
top: 138.0,
child: Column(
children: <Widget>[
Text(categorias.productName,
style: TextStyle(
fontFamily: 'BalooMedium',
fontSize: 10.0,
color: Colors.grey[200],
),),
Row(children: [
Icon(
Icons.star,
color: Colors.grey.withOpacity(0.5),
size: 15.0,
),
SizedBox(width: 3.0),
Text(categorias.productRating,
style: TextStyle(
fontFamily: 'BalooMedium',
fontSize: 10.0,
color: Colors.grey[200],
),)
])
],
))
],
)));
}
}

Answer 1

Replace
mainMenu().onMenuTap()
with
widget.onMenuTap()
Calling mainMenu() just creates a new instance of the mainMenu class without the corresponding information so onMenuTap() is null.
The widget property is the "State object's configuration [which] is the corresponding StatefulWidget instance." Source
So the widget property, when used in the State object, contains the information you need so onMenuTap() would contain the supplied argument.