revert AdditionalContentControl visibility

Author: MohammadHadi Attarieh

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.TreeListView.xaml

@@ -262,8 +262,7 @@
                             Grid.ColumnSpan="2"
                             Content="{TemplateBinding Content}"
                             ContentTemplate="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:TreeViewAssist.AdditionalTemplate)}"
-                            ContentTemplateSelector="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:TreeViewAssist.AdditionalTemplateSelector)}"
-                            Visibility="Collapsed" />
+                            ContentTemplateSelector="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:TreeViewAssist.AdditionalTemplateSelector)}" />
           </Grid>
           <ControlTemplate.Triggers>
             <DataTrigger Binding="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:TreeViewAssist.AdditionalTemplate), Converter={x:Static converters:NullableToVisibilityConverter.CollapsedInstance}, Mode=OneWay}" Value="Visible">