"Property elements cannot be in the middle of an element's content." error in WPF application

Question

I am using Dragablz tab control in my WPF application. Following code in my App.xaml was running fine last night but when I loaded the project today, it is showing me this error:

Property elements cannot be in the middle of an element's content. They must be before or after the content.

<Application x:Class="MVCP.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:dragablz="clr-namespace:Dragablz;assembly=Dragablz"
             StartupUri="FloatingActivator.xaml">
    <Application.Resources>

        <ResourceDictionary>
            <Style x:Key="OpenSans">
                <Setter Property="TextElement.FontFamily" Value="Open Sans, /MVCP;component/Fonts/#Open Sans" />
            </Style>
            <ResourceDictionary.MergedDictionaries>
                <!-- primary color -->
                <ResourceDictionary>
                    <!-- include your primary palette -->
                    <ResourceDictionary.MergedDictionaries>
                        <ResourceDictionary Source="pack://application:,,,/MaterialDesignColors;component/Themes/MaterialDesignColor.Indigo.xaml" />
                    </ResourceDictionary.MergedDictionaries>
                    <!--
                            include three hues from the primary palette (and the associated forecolours).
                            Do not rename, keep in sequence; light to dark.
                        -->
                    <SolidColorBrush x:Key="PrimaryHueLightBrush" Color="{StaticResource Primary100}"/>
                    <SolidColorBrush x:Key="PrimaryHueLightForegroundBrush" Color="{StaticResource Primary100Foreground}"/>
                    <SolidColorBrush x:Key="PrimaryHueMidBrush" Color="{StaticResource Primary500}"/>
                    <SolidColorBrush x:Key="PrimaryHueMidForegroundBrush" Color="{StaticResource Primary500Foreground}"/>
                    <SolidColorBrush x:Key="PrimaryHueDarkBrush" Color="{StaticResource Primary700}"/>
                    <SolidColorBrush x:Key="PrimaryHueDarkForegroundBrush" Color="{StaticResource Primary700Foreground}"/>
                </ResourceDictionary>

                <!-- secondary colour -->
                <ResourceDictionary>
                    <!-- include your secondary pallette -->
                    <ResourceDictionary.MergedDictionaries>
                        <ResourceDictionary Source="pack://application:,,,/MaterialDesignColors;component/Themes/MaterialDesignColor.Yellow.xaml" />
                    </ResourceDictionary.MergedDictionaries>

                    <!-- include a single secondary accent color (and the associated forecolour) -->
                    <SolidColorBrush x:Key="SecondaryAccentBrush" Color="{StaticResource Accent200}"/>
                    <SolidColorBrush x:Key="SecondaryAccentForegroundBrush" Color="{StaticResource Accent200Foreground}"/>
                </ResourceDictionary>

                <!-- Include the Dragablz Material Design style -->
                <ResourceDictionary Source="pack://application:,,,/Dragablz;component/Themes/materialdesign.xaml"/>

            </ResourceDictionary.MergedDictionaries>
            <!-- tell Dragablz tab control to use the Material Design theme -->
            <Style TargetType="{x:Type dragablz:TabablzControl}" BasedOn="{StaticResource MaterialDesignTabablzControlStyle}" />
            <Style x:Key="FileItemStyle" TargetType="{x:Type ListViewItem}">
                <Setter Property="Margin" Value="5,5,5,5"/>
                <Setter Property="Padding" Value="0,0,0,0"/>
                <Setter Property="HorizontalAlignment" Value="Left"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate  TargetType="{x:Type ListViewItem}">
                            <Grid HorizontalAlignment="Left" VerticalAlignment="Top" Height="50" >
                                <Border x:Name="border" BorderBrush="{x:Null}" BorderThickness="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" CornerRadius="2.5"/>
                                <StackPanel HorizontalAlignment="Stretch"  VerticalAlignment="Stretch">
                                    <ContentPresenter/>
                                </StackPanel>
                            </Grid>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>

        </ResourceDictionary>
    </Application.Resources>
</Application>

Answer 1

The reason is simple: you first add style "OpenSans", then you set MergedDictionary property, then you add two more styles. To fix, just reorder like this:

<Application x:Class="MVCP.App"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:dragablz="clr-namespace:Dragablz;assembly=Dragablz"
         StartupUri="FloatingActivator.xaml">
<Application.Resources>

    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <!-- primary color -->
            <ResourceDictionary>
                <!-- include your primary palette -->
                <ResourceDictionary.MergedDictionaries>
                    <ResourceDictionary Source="pack://application:,,,/MaterialDesignColors;component/Themes/MaterialDesignColor.Indigo.xaml" />
                </ResourceDictionary.MergedDictionaries>
                <!--
                        include three hues from the primary palette (and the associated forecolours).
                        Do not rename, keep in sequence; light to dark.
                    -->
                <SolidColorBrush x:Key="PrimaryHueLightBrush" Color="{StaticResource Primary100}"/>
                <SolidColorBrush x:Key="PrimaryHueLightForegroundBrush" Color="{StaticResource Primary100Foreground}"/>
                <SolidColorBrush x:Key="PrimaryHueMidBrush" Color="{StaticResource Primary500}"/>
                <SolidColorBrush x:Key="PrimaryHueMidForegroundBrush" Color="{StaticResource Primary500Foreground}"/>
                <SolidColorBrush x:Key="PrimaryHueDarkBrush" Color="{StaticResource Primary700}"/>
                <SolidColorBrush x:Key="PrimaryHueDarkForegroundBrush" Color="{StaticResource Primary700Foreground}"/>
            </ResourceDictionary>

            <!-- secondary colour -->
            <ResourceDictionary>
                <!-- include your secondary pallette -->
                <ResourceDictionary.MergedDictionaries>
                    <ResourceDictionary Source="pack://application:,,,/MaterialDesignColors;component/Themes/MaterialDesignColor.Yellow.xaml" />
                </ResourceDictionary.MergedDictionaries>
                <!-- include a single secondary accent color (and the associated forecolour) -->
                <SolidColorBrush x:Key="SecondaryAccentBrush" Color="{StaticResource Accent200}"/>
                <SolidColorBrush x:Key="SecondaryAccentForegroundBrush" Color="{StaticResource Accent200Foreground}"/>
            </ResourceDictionary>

            <!-- Include the Dragablz Material Design style -->
            <ResourceDictionary Source="pack://application:,,,/Dragablz;component/Themes/materialdesign.xaml"/>

        </ResourceDictionary.MergedDictionaries>
        <Style x:Key="OpenSans">
            <Setter Property="TextElement.FontFamily" Value="Open Sans, /MVCP;component/Fonts/#Open Sans" />
        </Style>
        <!-- tell Dragablz tab control to use the Material Design theme -->
        <Style TargetType="{x:Type dragablz:TabablzControl}" BasedOn="{StaticResource MaterialDesignTabablzControlStyle}" />
        <Style x:Key="FileItemStyle" TargetType="{x:Type ListViewItem}">
            <Setter Property="Margin" Value="5,5,5,5"/>
            <Setter Property="Padding" Value="0,0,0,0"/>
            <Setter Property="HorizontalAlignment" Value="Left"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate  TargetType="{x:Type ListViewItem}">
                        <Grid HorizontalAlignment="Left" VerticalAlignment="Top" Height="50" >
                            <Border x:Name="border" BorderBrush="{x:Null}" BorderThickness="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" CornerRadius="2.5"/>
                            <StackPanel HorizontalAlignment="Stretch"  VerticalAlignment="Stretch">
                                <ContentPresenter/>
                            </StackPanel>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

    </ResourceDictionary>
</Application.Resources>