'System.Windows.Media.SolidColorBrush' must have IsFrozen set to false

I have am using a DockPanel. It has a tool window container on the left and on the right and and MDI container in the middle. The tabs and windows that populate these 3 containers are generated dynamically.

It does not happen on a predictable basis but many times when I click on one of the auto hide tabs of the tool window to expand the tab, the program crashes with the following error:

Specified value of type 'System.Windows.Media.SolidColorBrush' must have IsFrozen set to false to modify.

Here is the xaml for the window.

            <DockPanel Margin="2,2,2,2" SnapsToDevicePixels="True">
                <DockPanel.Resources>
                    <converters:BooleanVisibilityConverter x:Key="BooleanVisibilityConverter" />
                </DockPanel.Resources>
                <Menu DockPanel.Dock="Top" SnapsToDevicePixels="True">
                    <MenuItem Header="File">
                        <MenuItem Header="New Patient Admission" Visibility="{Binding CanViewReports,Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsed}" Name="_newAdmissionMenuItem" Click="_newAdmissionMenuItem_Click" />
                        <Separator />
                        <MenuItem Header="Exit" Name="_exit" Click="_exit_Click" />
                        <Separator />
                        <MenuItem Header="Refresh My Security" Command="{Binding RefreshUserSecurityCommand}" />
                    </MenuItem>
                    <MenuItem Header="Reports" ItemsSource="{Binding ReportsMenuItems}">
                        
                    </MenuItem>
                    <MenuItem Header="Tools">
                        <MenuItem Header="Reassign Patients" Click="ReassignPatientsClick" />
                        <Separator />
                        <MenuItem Click="EditCrossCoveringPhysicians_click" Visibility="{Binding CanViewReports,Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsed}" Header="Cross Cover Schedule" />
                        <Separator />
                        <MenuItem Click="ImportMedicalProfessionals_Click" Visibility="{Binding CanViewReports,Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsed}" Header="Import Medical Professionals from Excel" />
                        <Separator />
                        <MenuItem Header="Calculator" Click="Calculator_Click" />
                        <Separator />
                        <MenuItem Header="Physician Phone Book" Click="PhysicianPhoneBook_Click"/>
                        <MenuItem Header="Manage Medical Professionals" Visibility="{Binding CanViewReports,Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsed}" Click="ManageMedicalProfessionals_Click"/>
                        <Separator />
                        <MenuItem Header="Light Manager" Visibility="{Binding CanViewReports,Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsed}" Click="LightManager_Click" />
                        <Separator />
                        <MenuItem Visibility="{Binding CanEnableEditingOfDischargePatients,Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsed}" Command="{Binding EnableEditingOfDischargedPatientsCommand}">
                            <MenuItem.Header>
                                <TextBlock Text="{Binding EditingOfDischargedPatientsMenuText,Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"/>
                            </MenuItem.Header>
                        </MenuItem>
                        <Separator />
                        <MenuItem Header="View Patient Lab Alerts for All Patients" Command="{Binding ViewAllPatientLabAlertCommand}" Visibility="{Binding CurrentUser.DisplayLabAlertIconOnAllPatients, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsefalse}" />
                        <MenuItem Header="View Patient Lab Alerts for only My Patients" Command="{Binding ViewOnlyMyAssignedPatientLabAlertCommand}" Visibility="{Binding CurrentUser.DisplayLabAlertIconOnAllPatients, Converter={StaticResource BooleanVisibilityConverter}, ConverterParameter=collapsed}" />
                    </MenuItem>
                    <MenuItem Header="Help">
                        <MenuItem Header="Index" Click="ShowHelpIndex" />
                        <Separator />
                        <MenuItem Click="ShowAboutDialog" Header="About PediNotes"/>
                    </MenuItem>
                </Menu>
                <Grid SnapsToDevicePixels="True">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto" />
                        <RowDefinition Height="*" />
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*"/>
                    </Grid.ColumnDefinitions>

                    <my:SourceObjectContainer Background="#293955" SnapsToDevicePixels="True" x:Name="_sourceObjectContainer">

                    </my:SourceObjectContainer>
                    <Border Grid.Row="1"  BorderThickness="20,0,20,25" BorderBrush="#293955" Background="{DynamicResource VSBackgroundBrush}" >
                        <!--<Rectangle Fill="{DynamicResource KidsNoteImage}" Width="350" Height="230" />-->
                        <Rectangle Fill="{DynamicResource InfanotesMainScreenImage}" Width="350" Height="230" />
                    </Border>
                    <docking:DockSite x:Name="_dockSite" Background="Transparent" Grid.Row="1" CanDocumentWindowsRaft="True" SnapsToDevicePixels="True" Foreground="White" >
                        <docking:DockSite.AutoHideLeftContainers>
                            <docking:ToolWindowContainer x:Name="_sourceItemsToolWindowContainer" docking:DockSite.ControlSize="370,500" TitleBarForegroundActive="White" TitleBarForegroundInactive="White" TitleBarBackgroundActive="{DynamicResource VSBackgroundBrush}" TitleBarBackgroundInactive="{DynamicResource VSBackgroundBrush}" SnapsToDevicePixels="True"/>
                        </docking:DockSite.AutoHideLeftContainers>
                        <docking:Workspace Background="Transparent">
                            <docking:TabbedMdiHost x:Name="_tabbedMDIHost" SnapsToDevicePixels="True" Background="{DynamicResource VSBackgroundBrush}" Foreground="White" TabPlacement="Top" IsCloseButtonOnTab="True" TabOverflowBehavior="ScrollWithMenu">
                                <docking:TabbedMdiContainer BorderThickness="0" SnapsToDevicePixels="True">

                                </docking:TabbedMdiContainer>
                            </docking:TabbedMdiHost>
                        </docking:Workspace>
                        <docking:DockSite.AutoHideRightContainers>
                            <docking:ToolWindowContainer x:Name="_alertItemsToolWindowContainer" TitleBarForegroundActive="White" TitleBarForegroundInactive="White" TitleBarBackgroundActive="{DynamicResource VSBackgroundBrush}" TitleBarBackgroundInactive="{DynamicResource VSBackgroundBrush}"/>
                        </docking:DockSite.AutoHideRightContainers>
                    </docking:DockSite>
                </Grid>
            </DockPanel>