filling Grid panel from Store doesn't work EXTJS 4.2 - extjs4.2

Question

Good Morning,
I have some problem when I attempted to fill my Grid panel from my web service, the result was an empty grid, thank you for advance.
// Create store
var myStore = new Ext.data.JsonStore({
proxy: new Ext.data.HttpProxy({
url: 'Service.asmx/GetPeople',
headers: {
'Content-type': 'application/json'
}
}),
root: 'd',
id: 'Id',
fields: ['Id', 'FistName', 'LastName', 'BirthDate'],
autoLoad: true
});
//Create grid to display data from store
var gridTest = new Ext.grid.Panel({
store: myStore, // Our store
renderTo: Ext.getBody(),
forceFit: true,
columns: [ // Grid columns
{ xtype: 'gridcolumn', width: 100, text: 'Id', dataIndex: 'Id', flex: 1 },
{ xtype: 'gridcolumn', width: 100, text: 'Nom', dataIndex: 'FirstName', flex: 1 },
{ xtype: 'gridcolumn', width: 100, text: 'Mail', dataIndex: 'LastName', flex: 1 },
{ xtype: 'gridcolumn', width: 100, text: 'Phone', dataIndex: 'BirthDate', flex: 1 }
]
});
and this is my viewport:
Ext.create('Ext.container.Viewport', {
layout: "border",
items: [{
region: "north",
height: 50,
title: "Nord"
}, {
region: "south",
height: 200,
title: "sud",
bodyStyle: 'background: #fffff;',
border: false
}, {
region: "center",
title: "centre",
bodyStyle: 'background: #00000;',
border: false,
items: gridTest
}, {
region: "west",
width: 100,
title: "ouest"
}, {
region: "east",
width: 100,
title: "est"
}]
});
and this is RESPONSE FROM FIREBUG MOZILLA FIREFOX:
{"d": [{
"__type":"WebService4ExtJS.Model.Person",
"Id":0,
"FirstName":"sami",
"LastName":"samibizani#gmail.com",
"BirthDate":"23188219"
}, {
"__type":"WebService4ExtJS.Model.Person",
"Id":1,"FirstName":"admin",
"LastName":"admin#gmail.com",
"BirthDate":"1111111"
}, {
"__type":"WebService4ExtJS.Model.Person",
"Id":2,
"FirstName":"user",
"LastName":"user#gmail.com",
"BirthDate":"2222222"
}]
}

Answer 1

Your Store reader is not defined:
reader: {
type: 'json',
root: 'd',
idProperty: 'Id'
}
You can find the working code here.
And one little mistake: in the fields array, you wrote FistName instead of FirstName.