How do I resolve the error: value of type cannot be added to a collection or dictionary of type 'UIElementCollection'

[ChrisSergent]

I have a xaml file with the following code and I receive an error for my local:TextEditorToolbar:

Window x:Class="TextEditor.MainWindow"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

xmlns:local="clr-namespace:TextEditor"

Title="Text Editor" Height="600" Width="800">

<DockPanel>

<Menu x:Name="menu"

DockPanel.Dock="Top"/>

<StackPanel>

<local:TextEditorToolbar x:Name="toolbar"/>

</StackPanel>

<StatusBar DockPanel.Dock="Bottom">

<TextBlock x:Name="status" />

</StatusBar>

<RichTextBox x:Name="body"

SpellCheck.IsEnabled="True"

AcceptsTab="True"

AcceptsReturn="True"

BorderThickness="0 2 0 0" />

</DockPanel>

Window>

It references a user control name TextEditorToolbar.xaml. Why would I receive this error and how do I fix it.