Unable to insert tabstop in empty last line

I am having this problem as well.  The use the code below to reproduce the error. 

The project references:

ActiproSoftware.SyntaxEditor.Wpf.dll version 15.1.624.0

<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:editor="http://schemas.actiprosoftware.com/winfx/xaml/syntaxeditor"
Title="Syntax Editor" >
    <Grid>
        <editor:SyntaxEditor />
    </Grid>
</Window>

Thanks for reporting this.  It was caused by an oversight in the change we made for this thread:

https://www.actiprosoftware.com/community/thread/23217/indent-does-not-work-on-last-selected-line#111252

We've made a hotfix for the current production build that fixes this.  If you redownload it now, you should have the hotfix.