I just want to save a normal TextBox widget to another variable, so as soon as the error occurs

System.NullReferenceException: “The object reference was not set to an object instance.”

you can even see that everything has a value and none of these variables is null

private void AddMessageButton_Click(object sender, RoutedEventArgs e)
{
    Schools_ViewModel viewModel = new Schools_ViewModel();

    string subject = SubjectTextBox.Text.Trim();
    string messageText = MessageTextBox.Text.Trim();

    int? userId = UserSession.Instance.CurrentUser.UserId; 
    int? teacherId = viewModel.selectedTeacher.TeacherId;
    int? fileId = null;

    viewModel.AddMessage(userId, teacherId, fileId, subject, messageText);

    SubjectTextBox.Clear();
    MessageTextBox.Clear();
}

and here is the view

    <StackPanel Grid.Row="1">
    <Border BorderBrush="#b9bbbe" BorderThickness="1" Margin="5">
        <TextBox x:Name="SubjectTextBox" Text="Enter Subject" BorderThickness="0" 
             Background="Transparent" Foreground="#ffffff" Height="30" AcceptsReturn="True" 
             TextWrapping="Wrap"/>
    </Border>
    <Border BorderThickness="0, 1, 0, 1" BorderBrush="#b9bbbe" Margin="5, 0, 5, 0">
        
        <TextBox x:Name="MessageTextBox" Text="Enter Message" Margin="5" BorderThickness="0" 
                 Background="Transparent" Foreground="#ffffff" Height="150" AcceptsReturn="True" 
                 TextWrapping="Wrap"/>
    </Border>
    <TextBlock Name="fileInfoTextBlock" Foreground="#ffffff" Margin="5" FontSize="13"/>
</StackPanel>

idk what to try next