Can't access mainwindow class from page?

Question

i have wpf app which have a mainwindow.xaml and a pages/page1.xaml .Page display on window using frame .window contain a textbox and when i try to give value to this textbox from page1 on button click then nothing happen in textbox . so what should i do?

i am using that code on button click-
Mainwindow.xaml
<Window x:Class="pp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:v="clr-namespace:pp.Pages"
        Title="MainWindow" MinHeight="350" MinWidth="525"
           Background="Transparent">
 <Grid>
       <TextBox  Width="100" Height="40" Name="textbox1"/>
 <Frame NavigationUIVisibility="Hidden" Name="Mainframe" Source="pages/page1.xaml" Margin="40"></Frame>
 </Grid>
</Window>


Page1.xaml
<Page x:Class="pp.FileView"
      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:v="clr-namespace:pp.Pages"
      mc:Ignorable="d" 
      d:DesignHeight="300" d:DesignWidth="300"
Title="FileView">


    <Grid >
        <Button Width="100" Height="40" Name="Btn" Click="Btn_click"/>   
     </Grid>
</page>


Page1.xaml.cs
        private void Btn_Click(object sender, RoutedEventArgs e)
        {
        MainWindow mw=new MainWindow();
        mw.textbox1.Text="hello";
 }