WPF MVVM Problem in TabItems

<usercontrol x:class="MVVMTestApplication.Views.CustomerView" xmlns:x="#unknown">
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:vm="clr-namespace:MVVMTestApplication.ViewModels"
             mc:Ignorable="d" FontFamily="Tahoma" 
             d:DesignHeight="431" d:DesignWidth="483" FocusManager.FocusedElement="{Binding txtname}" xmlns:my="clr-namespace:Navigation;assembly=Navigation">
    <usercontrol.resources>
        <vm:customerviewmodel x:key="CVM" xmlns:vm="#unknown" />
        <collectionviewsource x:key="CVS" />
    </usercontrol.resources>
    
    <usercontrol.inputbindings>
        <keybinding key="N" modifiers="Control" command="{Binding AddNewCommadn}" />
        <keybinding key="S" modifiers="Control" command="{Binding InsertCommand}" />
    </usercontrol.inputbindings>
    
    <grid flowdirection="RightToLeft" datacontext="{Binding Source={StaticResource CVM}}">
        <tabcontrol background="Wheat" margin="3,41,3,3">
            <tabitem header="Review">
                <datagrid canuseraddrows="False" itemssource="{Binding CVS.View}" selecteditem="{Binding CurrentCustomer}" autogeneratecolumns="False" margin="10" name="dataGrid1" background="White">
                    <datagrid.columns>
                        <datagridtextcolumn header="Name" binding="{Binding CustomerName,UpdateSourceTrigger=PropertyChanged}" width="100" />
                        <datagridtextcolumn header="Family" binding="{Binding Path=CustomerFamily,UpdateSourceTrigger=PropertyChanged}" width="100" />
                        <datagridtextcolumn header="Phone" binding="{Binding Path=CustomerPhone,UpdateSourceTrigger=PropertyChanged}" width="100" />
                    </datagrid.columns>
                </datagrid>
            </tabitem>
            <tabitem header="Edit Section">
                <groupbox>
                    <grid datacontext="{Binding CurrentCustomer}">
                        <textbox height="23" text="{Binding Path=CustomerName,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" horizontalalignment="Left" margin="70,18,0,0" name="txtname" verticalalignment="Top" width="120" inputlanguagemanager.inputlanguage="fa-ir" />
                        <textbox height="23" text="{Binding Path=CustomerFamily,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" horizontalalignment="Left" margin="70,47,0,0" name="textBox2" verticalalignment="Top" width="120" />
                        <textbox height="23" text="{Binding Path=CustomerPhone,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" horizontalalignment="Left" margin="70,76,0,0" name="textBox3" verticalalignment="Top" width="120" />
                        <textblock height="23" horizontalalignment="Left" margin="12,18,0,0" name="textBlock1" text="Name :" verticalalignment="Top" />
                        <textblock height="23" horizontalalignment="Left" margin="12,47,0,0" name="textBlock2" text="Family :" verticalalignment="Top" />
                        <textblock height="23" horizontalalignment="Left" margin="12,76,0,0" name="textBlock3" text="Phone :" verticalalignment="Top" />



        <button command="{Binding AddNewCommand}" commandparameter="{Binding ElementName=txtname,Path=Text}" content="Insert" height="23" horizontalalignment="Left" margin="238,12,0,0" name="button2" verticalalignment="Top" width="75" />
        <button command="{Binding InsertCommand}" content="Save" height="23" horizontalalignment="Left" margin="319,12,0,0" name="button1" verticalalignment="Top" width="75" />
                    </grid>
                </groupbox>

            </tabitem>
        </tabcontrol>
        <!--<Button Command="{Binding NewxItemCommand}" Content="Next" Height="23" HorizontalAlignment="Left" Margin="55,4,0,0" Name="button3" VerticalAlignment="Top" Width="75" />-->
        <!--<Button Command="{Binding BackItemCommand}" Content="Back" Height="23" HorizontalAlignment="Left" Margin="136,4,0,0" Name="button4" VerticalAlignment="Top" Width="75" />-->


    </grid>
</usercontrol>