Without making a new epub,
if you just open an HTML file and then save it as your new epub.
In this case, there is no toc.ncx in your epub.
So if you add some head tags on your document for making a TOC(Table of Contents) and then 'Generate Table of Contents', Sigil crashes
. Because there is no toc.ncx file in your epub.
<solution1>
Without making a new epub,
when a user just opens an HTML file and then saves it as his/her new epub,
let Sigil automatically make and add a toc.ncx frame file in the epub.
<solution 2>
When Sigil generates a Table of Contents without the file toc.ncx,
it has to forcefully make toc.ncx file first and then generate TOC.