Issue for loading XAML layout inline as a string

I’m using pyrevit, and my WPF form is not being displayed (no error appears on the screen). The XAML layout is defined inline as a string. Interestingly, in another form where the XAML layout is also defined inline, the form is displayed correctly!?

# -*- coding: UTF-8 -*-
from pyrevit import forms
import wpf, clr

clr.AddReference("PresentationFramework")
clr.AddReference("System.Xml")
clr.AddReference("PresentationCore")
clr.AddReference("System.Windows")

from System.IO import StringReader
from System.Windows.Markup import XamlReader
from System.Windows import Window
from pyrevit.forms import WPFWindow

class Time(Window):
    LAYOUT = '''
        <Window 
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            Title="MainWindow" Height="450" Width="800">
            <Grid >
            </Grid>
        </Window>'''

    def __init__(self):
        wpf.LoadComponent(self, StringReader(Time.LAYOUT))
        self.ShowDialog()