Able to return a string from the async method

Fixed ObservableObject

WpfViewModelFirst/MainWindow.xaml

@@ -4,9 +4,31 @@
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:local="clr-namespace:WpfViewModelFirst"
+        xmlns:part1="clr-namespace:WpfViewModelFirst.Part1"
+        xmlns:part2="clr-namespace:WpfViewModelFirst.Part2"
         mc:Ignorable="d"
+        d:DataContext="{d:DesignInstance Type={x:Type local:MainWindowViewModel}}"
         Title="MainWindow" Height="450" Width="800">
-    <Grid>
-
+    <Grid Background="DarkRed"> 
+        <Grid.RowDefinitions>
+            <RowDefinition/>
+            <RowDefinition Height="auto"/>
+        </Grid.RowDefinitions>
+        <ContentControl Grid.Row="0" 
+                        Visibility="Visible"
+                        Content="{Binding CurrentViewModel}">
+            <ContentControl.Resources>
+                <DataTemplate DataType="{x:Type part1:Part1ViewModel}">
+                    <part1:Part1View/>
+                </DataTemplate>
+                <DataTemplate DataType="{x:Type part2:Part2ViewModel}">
+                    <part2:Part2View/>
+                </DataTemplate>
+            </ContentControl.Resources>
+        </ContentControl>
+        <Button Grid.Row="1" 
+                HorizontalAlignment="Center"
+                Command="{Binding StartCommand}" 
+                Content="Start"/>
     </Grid>
 </Window>