www.makemkv.com

I have one mkv file with 15 chapters.
When I run 'mkvextract' all the chapters are shown like:

<?xml version="1.0"?>
<!-- <!DOCTYPE Chapters SYSTEM "matroskachapters.dtd"> -->
<Chapters>
<EditionEntry>
<EditionUID>801521038</EditionUID>
<EditionFlagDefault>0</EditionFlagDefault>
<EditionFlagHidden>0</EditionFlagHidden>
<ChapterAtom>
<ChapterUID>280084908</ChapterUID>
<ChapterTimeStart>00:00:00.000000000</ChapterTimeStart>
<ChapterFlagHidden>0</ChapterFlagHidden>
<ChapterFlagEnabled>1</ChapterFlagEnabled>
<ChapterDisplay>
<ChapterString>Overture</ChapterString>
<ChapterLanguage>und</ChapterLanguage>
</ChapterDisplay>
</ChapterAtom>
...

but when I try to remove some chapters with 'mkvmerge GUI' all in tab 'Chapter Editor' are grayed out and no chapters are shown.
I have Linux Mint 3.2.0-4-amd64 and MKVToolNix 6.1.0
How can remove some chapters ?

Wouldn't it seem this is a question more appropriate for the MKVToolnix website?