I had this same error for a while in a slightly different scenario. The cause was invalid XAML, because some tags were missing.
I had
<wpftoolkit:DataGrid
AutoGenerateColumns="False"
ItemsSource="{Binding Path=Accounts}" >
<wpftoolkit:DataGridTextColumn
Header="Account Name"
Binding="{Binding Path=AccountName}" />
</wpftoolkit:DataGrid>
which I fixed to be
<wpftoolkit:DataGrid
AutoGenerateColumns="False"
ItemsSource="{Binding Path=Accounts}" >
<wpftoolkit:DataGrid.Columns>
<wpftoolkit:DataGridTextColumn
Header="Account Name"
Binding="{Binding Path=AccountName}" />
</wpftoolkit:DataGrid.Columns>
</wpftoolkit:DataGrid>