[Urgent] C# WPF Help

[OmegaStarScream]

Bounty is negotiable.

I will try to explain the situation as much as possible, feel free to ask any questions If you need further info.

I'm using the following project: https://github.com/ButchersBoy/MaterialDesignInXamlToolkit

It contains the demo, I'm simply trying to copy the demo into my own project. So far, I'm doing good but I stumbled upon an issue. I'm using ReSharper Extension and It gave me an error (under "DemoItems" on the second line):

Code:

 <ListBox x:Name="DemoItemsListBox" Margin="0 16 0 16" SelectedIndex="0"                         
                             ItemsSource="{Binding DemoItems}"
                             PreviewMouseLeftButtonUp="UIElement_OnPreviewMouseLeftButtonUp">


                        <ListBox.ItemTemplate>
                            <DataTemplate DataType="domain:DemoItem">
                                <TextBlock Text="{Binding Name}" Margin="32 0 32 0" />
                            </DataTemplate>
                        </ListBox.ItemTemplate>
                    </ListBox>

The error message (I can compile but nothing fills the Listbox): Cannot resolve symbol "DemoItems" Due to unknown dataContext
The file in question is MainWindow.xaml: https://github.com/ButchersBoy/MaterialDesignInXamlToolkit/blob/master/MainDemo.Wpf/MainWindow.xaml

If you're good, I believe you won't even need to download the source code.

The weird thing here is that I downloaded that demo source code, I have the same error messages there, but when I compile, the Listbox gets filled with data and that's not the case for my application.