WPF usercontrol shows configuration error - wpf

Question

I have a simple UserControl
<UserControl x:Class="UIWpf.UserControls.MultiSelectEnvironmentContextControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="500" d:DesignWidth="500">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="25*"/>
<RowDefinition Height="30*"/>
<RowDefinition Height="31*"/>
<RowDefinition Height="29*"/>
<RowDefinition Height="36*"/>
<RowDefinition Height="149*"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="149*"/>
<ColumnDefinition Width="151*"/>
</Grid.ColumnDefinitions>
<ComboBox Grid.Column="0" Grid.Row="0" x:Name="ComboBoxVersions" SelectedIndex="0">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
<StackPanel Orientation="Horizontal" Grid.Column="1" Grid.Row="0">
<TextBlock >Tests to be run for:</TextBlock>
<ComboBox Name="ComboBoxFileTypeSelector" DisplayMemberPath="Key" SelectedValuePath="Value" SelectedIndex="0">
</ComboBox>
</StackPanel>
</Grid>
</UserControl>
Now, I am using this UserControl in the MainWindow.xaml
<Window x:Class="UIWpf.MainWindow"
xmlns:u="clr-namespace:UIWpf.UserControls"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition></ColumnDefinition>
<ColumnDefinition></ColumnDefinition>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition>
</RowDefinition>
</Grid.RowDefinitions>
<u:MultiSelectEnvironmentContextControl x:Name="MultiSelectEnvironmentContextControl"></u:MultiSelectEnvironmentContextControl>
</Grid>
</Window>
The UserControl uses data binding and data comes from a source which is configured in the config file.
I have the configuration settings in the app.config and when I run the application, the MainForm works fine with the UserControl populated with the data.
But at compile time I still see an error saying , the setting is missing from the configuration file.