Windows workflow activity: controls vanishing after collapsing

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WWF

workflow

I have a windows workflow designer wherein the customer can design his own workflows. I have created a custom workflow activity names FinalizeParticipantWorkflow which looks like this:

http://postimg.org/image/6xvi8zse3/[^]

Clicking on the checkbox disables both the dropdowns.

My problem is when I am selecting a value from the First dropdown and then collapsing the container and then again expand it, all the controls vanish from the container. Like this:

http://postimg.org/image/ri0c1ktl3/[^]

Here is the xaml code for the activity:

<pre lang="HTML"><sap:ActivityDesigner x:Class="MMS.Activities.Public.Design.FinalizeParticipantWorkflowDesigner"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:s="clr-namespace:System;assembly=mscorlib"
    xmlns:sap="clr-namespace:System.Activities.Presentation;assembly=System.Activities.Presentation"
    xmlns:sapv="clr-namespace:System.Activities.Presentation.View;assembly=System.Activities.Presentation"
    xmlns:sapc="clr-namespace:System.Activities.Presentation.Converters;assembly=System.Activities.Presentation"
    xmlns:Model="clr-namespace:System.Activities.Presentation.Model;assembly=System.Activities.Presentation"
                      xmlns:ActivitiesLibrary="clr-namespace:MMS.Activities.Common;assembly=MMS.Activities"
                      xmlns:et="clr-namespace:MMS.EventEngine.Entities;assembly=MMS.EventEngine.Entities"
                      xmlns:grp="clr-namespace:MMS.Activities.Public.Design"
                      xmlns:local="clr-namespace:MMS.Activities.Public.Design.Converters" Loaded="ActivityDesigner_Loaded">
    <sap:ActivityDesigner.Resources>

        <ResourceDictionary x:Uid="ResourceDictionary_1">
            <sapc:ArgumentToExpressionConverter x:Key="ArgumentToExpressionConverter" />
            <sapc:ModelToObjectValueConverter x:Key="ModelToObjectValueConverter" />
            <local:StringConverter x:Key="StringConverter" />
            <local:InverseBooleanConverter x:Key="InverseBooleanConverter" />
            <local:BooleanConverter x:Key="BooleanConverter"/>

            <ObjectDataProvider x:Key="AssociatedGroupsDs" MethodName="GetAssociatedGroups" ObjectType="{x:Type grp:FinalizeParticipantWorkflowDesigner}"></ObjectDataProvider>
            <!--<ObjectDataProvider x:Key="AssociatedGroupsDs1" MethodName="GetAssociatedGroups" ObjectType="{x:Type grp:FinalizeParticipantWorkflowDesigner}"></ObjectDataProvider>-->
            <ObjectDataProvider x:Key="SuccessorWorkflowIds" MethodName="GetSuccessorWorkflowIds" ObjectType="{x:Type grp:FinalizeParticipantWorkflowDesigner}"></ObjectDataProvider>

            <DataTemplate x:Key="Collapsed">
                <StackPanel Orientation="Vertical">
                    <StackPanel Orientation="Horizontal">
                        <TextBlock VerticalAlignment="Center" Width="125" Margin="5" Text=""/>
                        <CheckBox x:Name="chkGetNextGroupByScore" IsChecked="{Binding Path=ModelItem.GetNextGroupByScore, Mode=TwoWay, Converter={StaticResource BooleanConverter}}" Content="Use Next Group Based On Score" Height="16" HorizontalAlignment="Left" Grid.Row="0" Grid.Column="1" Margin="5"  VerticalAlignment="Top" />
                    </StackPanel>
                </StackPanel>
            </DataTemplate>

            <DataTemplate x:Key="Expanded">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition />
                        <ColumnDefinition />
                    </Grid.ColumnDefinitions>

                    <Grid.RowDefinitions>
                        <RowDefinition />
                        <RowDefinition />
                        <RowDefinition />
                    </Grid.RowDefinitions>
                    <CheckBox x:Name="chkGetNextGroupByScore" IsChecked="{Binding Path=ModelItem.GetNextGroupByScore, Mode=TwoWay, Converter={StaticResource BooleanConverter}}" Content="Use Next Group Based On Score" Height="16" HorizontalAlignment="Left" Grid.Row="0" Grid.Column="1" Margin="5"  VerticalAlignment="Top" />
                    <TextBlock Width="125" VerticalAlignment="Center" Margin="5" Text="Successor Group Id"  Grid.Row="1" Grid.Column="0"/>
                    <ComboBox x:Name="cbSuccessorGroupId" IsEnabled="{Binding ElementName=chkGetNextGroupByScore, Path=IsChecked, Converter={StaticResource ResourceKey=InverseBooleanConverter}}" ItemsSource="{Binding Source={StaticResource AssociatedGroupsDs}}" SelectedValue="{Binding Path=ModelItem.SuccessorGroupId, Mode=TwoWay, Converter={StaticResource StringConverter}}" VerticalAlignment="Center" Width="300" Grid.Row="1" Grid.Column="1" Margin="0,5" SelectionChanged="ComboBox_SelectionChanged"></ComboBox>
                    <TextBlock Width="125" VerticalAlignment="Center" Margin="5" Text="Successor Workflow Id"  Grid.Row="2" Grid.Column="0"/>
                    <ComboBox x:Name="cbSuccessorWFs" IsEnabled="{Binding ElementName=chkGetNextGroupByScore, Path=IsChecked, Converter={StaticResource ResourceKey=InverseBooleanConverter}}" ItemsSource="{Binding Source={StaticResource SuccessorWorkflowIds}}" SelectedValue="{Binding Path=ModelItem.SuccessorWorkflowId, Mode=TwoWay, Converter={StaticResource StringConverter}}" VerticalAlignment="Center" Width="300" Grid.Row="2" Grid.Column="1" Margin="0,5"></ComboBox>
                </Grid>
            </DataTemplate>

            <Style x:Key="ExpandOrCollapsedStyle" TargetType="{x:Type ContentPresenter}">
                <Setter Property="ContentTemplate" Value="{DynamicResource Expanded}" />
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Path=ShowExpanded}" Value="false">
                        <Setter Property="ContentTemplate" Value="{DynamicResource Collapsed}" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>


        </ResourceDictionary>
    </sap:ActivityDesigner.Resources>
    <Grid>
        <ContentPresenter Style="{DynamicResource ExpandOrCollapsedStyle}" Content="{Binding}" />
    </Grid>
</sap:ActivityDesigner>

And here is the code for the dropdown change event:

private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            List<string> swf = new List<string>();
            swf = m.GetSuccessWorkFlowsByGroupList(this, ((ComboBox)sender).SelectedValue.ToString());
            foreach (var cb in FindVisualChildren<ComboBox>(this))
            {
                if (cb.Name == "cbSuccessorWFs")
                {
                    cb.ItemsSource = swf;
                }
            }
        }

If I do not select the value from dropdown, the expand/collapse works fine. It only breaks if I select the dropdown value.

I have searched all over the internet but could not find a solution. Can anyone suggest a remedy.