<NavBarGroupHeader>
  <ExplorerBarExpandButton>
    <Path>
      <SolidColorBrush/>
    </Path>
  </ExplorerBarExpandButton>
</NavBarGroupHeader>

<ControlTemplate TargetType="{x:Type dxn:NavBarGroupHeader}>
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition/>
      <ColumnDefinition Width="Auto"/>
    </Grid.ColumnDefinition>

    <VisualStateManager.VisualStateGroups>
      <VisualStateGroup x:Name="Common">
        <VisualState x:Name="Normal"/>
        <VisualState x:Name="MouseOver">
          <Storyboard>
            <!-- these two lines throw the exception -->
            <ColorAnimation Storyboard.Target="{Binding ElementName=ArrowTop}" Storyboard.TargetProperty="(Path.Fill).(SolidColorBrush.Color)" Duration="0" To="#201F35"/>
            <ColorAnimation Storyboard.Target="{Binding ElementName=ArrowBottom}" Storyboard.TargetProperty="(Path.Fill).(SolidColorBrush.Color)" Duration="0" To="#201F35"/>
          </Storyboard>
        </VisualState>
      </VisualStateGroup>
    </VisualStateManager.VisualStateGroup>

    <!-- This shows a text within the header -->
    <dxn:ImageAndTextContentPresenter/>

    <dxn:ExplorerBarExpandButton x:Name="ExplorerBarExpandButton"
                                 Grid.Column="1"
                                 Template="{StaticResource GroupBoxExpandButtonTemplate}"/>

    <dxn:NavPaneExpandButton x:Name="NavPaneExpandButton"
                             Grid.Column="1"
                             Template="{StaticResource GroupBoxExpandButtonTemplate}"/>
  </Grid>
</ControlTemplate>

<ControlTemplate x:Key="GroupBoxExpandButtonTemplate">
  <Grid>
    <VisualStateManager.VisualStateGroups>
      <VisualStateGroup x:Name="CommonStates">
        <VisualState x:Name="Normal"/>
      </VisualStateGroup>
      <VisualStateGroup x:Name="ExpandStates">
        <VisualState x:Name="Expanded"/>
        <VisualState x:Name="Collapsed">
          <Storyboard>
            <ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphClose" Storyboard.TargetProperty="(Control.Visibility)" dxcn:ValueSetter.Visibility="Collapsed"/>
            <ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphOpen" Storyboard.TargetProperty="(Control.Visibility)" dxcn:ValueSetter.Visibility="Visible"/>
          </Storyboard>
        </VisualState>
      </VisualStateGroup>
    </VisualStateManager.VisualStateGroups>

    <ContentControl x:Name="GlyphClose"
                    Template="{StaticResource CollapseExpandIconTemplate}"/>
    <ContentControl x:Name="GlyphOpen"
                    Template="{StaticResource CollapseExpandIconTemplate}">
      <ContentControl.LayoutTransform>
        <RotateTransform Angle="180"/>
      </ContentControl.LayoutTransform>
    </ContentControl>
  </Grid>
</ControlTemplate>

<ControlTemplate x:Key="CollapseExpandIconTemplate">
  <StackPanel>
    <Path x:Name="ArrowTop" Stretch="Uniform" Fill="#80838F" Data="M 0,50 25,50 50,30 75,50 100,50 100,40 50,0 0,40 Z"/>
    <Path x:Name="ArrowBottom" Stretch="Uniform" Fill="#80838F" Data="M 0,50 25,50 50,30 75,50 100,50 100,40 50,0 0,40 Z"/>
  </StackPanel>
</ControlTemplate>

</Path ...>
  <Path.Fill>
    <SolidColorBrush Color="#80838F"/>
  </Path.Fill>
</Path>

<ControlTemplate TargetType="{x:Type dxn:NavBarGroupHeader}>
  <Grid x:Name="HeaderRoot">
    <Grid.Background>
      <SolidColorBrush Color="White"/>
    </Grid.Background>

    <Grid.ColumnDefinitions>
      <ColumnDefinition/>
      <ColumnDefinition Width="Auto"/>
    </Grid.ColumnDefinition>

    <VisualStateManager.VisualStateGroups>
      <VisualStateGroup x:Name="Common">
        <VisualState x:Name="Normal"/>
        <VisualState x:Name="MouseOver">
          <Storyboard>
            <ColorAnimation Storyboard.Target="{Binding ElementName=HeaderRoot}" Storyboard.TargetProperty="(Grid.Background).(SolidColorBrush.Color)" Duration="0" To="#201F35"/>
          </Storyboard>
        </VisualState>
      </VisualStateGroup>
    </VisualStateManager.VisualStateGroup>

    <!-- This shows a text within the header -->
    <dxn:ImageAndTextContentPresenter/>

    <dxn:ExplorerBarExpandButton x:Name="ExplorerBarExpandButton"
                                 Grid.Column="1">
      <dxn:ExplorerBarExpandButton.Template>
        <ControlTemplate x:Key="GroupBoxExpandButtonTemplate">
          <Grid x:Name="ExpandButtonRoot">
            <VisualStateManager.VisualStateGroups>
              <VisualStateGroup x:Name="CommonStates">
                <VisualState x:Name="Normal"/>
              </VisualStateGroup>
              <VisualStateGroup x:Name="ExpandStates">
                <VisualState x:Name="Expanded"/>
                <VisualState x:Name="Collapsed">
                  <Storyboard>
                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphClose" Storyboard.TargetProperty="(Control.Visibility)" dxcn:ValueSetter.Visibility="Collapsed"/>
                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphOpen" Storyboard.TargetProperty="(Control.Visibility)" dxcn:ValueSetter.Visibility="Visible"/>
                  </Storyboard>
                </VisualState>
              </VisualStateGroup>
            </VisualStateManager.VisualStateGroups>

            <ContentControl x:Name="GlyphClose">
              <ContentControl.Template>
                <ControlTemplate>
                  <StackPanel x:Name="PathRoot">
                    <Path x:Name="ArrowBottom" Stretch="Uniform" Fill="#80838F" Data="M 0,50 25,50 50,30 75,50 100,50 100,40 50,0 0,40 Z"/>
                  </StackPanel>
                </ControlTemplate>
              </ContentControl.Template>
            </ContentControl>
          </Grid>
        </ControlTemplate>
      </dxn:ExplorerBarExpandButton.Template>
    </dxn:ExplorerBarExpandButton>
  </Grid>
</ControlTemplate>

<ColorAnimation Storyboard.Target="{Binding ElementName=ExpandButtonRoot}" Storyboard.TargetProperty="(Grid.Background).(SolidColorBrush.Color)" Duration="0" To="#201F35"/>