WPF - Resource Dictionary - "the resource could not be resolved"

<UserControl.Resources>      
    
    <converters:InverseBoolToVisibilityConverter x:Key="inverseBoolToVisibilityConverter" />
    <converters:EmptyStringToVisibilityConverter x:Key="emptyStringToVisibilityConverter" />
    
    <styleSelectors:TreeViewItemStyleSelector x:Key="treeViewItemStyleSelector" />
    
    <ResourceDictionary x:Key="MainDictionary">
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/GistManager;component/Styles/ToolbarStyles.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>

    <CollectionViewSource x:Key="privateGistsSource" 
                          Source="{Binding Gists}"
                          behaviors:CollectionViewSourceFilterBehavior.FilterCommand="{Binding PrivateFilterCommand}"
                          behaviors:CollectionViewSourceFilterBehavior.FilterTextBox="{Binding ElementName=searchBox}" />
</UserControl.Resources>

 <Separator Style="{StaticResource toolBarSeparatorStyle}"/>

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style x:Key="toolBarSeparatorStyle" TargetType="Separator" >
        <Setter Property="Background" Value="gray"></Setter>
        <Setter Property="Margin" Value="4"></Setter>
    </Style>
</ResourceDictionary>