KeyNotFoundException is thrown when importing RTF document where single font info is not declared in group, for example the following RTF doesn't work:
{\rtf1\ansi\ansicpg1250\deff0{\fonttbl\f0\fswiss Helvetica;}\f0\pard test\par}

while the following works:
{\rtf1\ansi\ansicpg1250\deff0{\fonttbl{\f0\fswiss Helvetica;}}\f0\pard test \par}

This is valid according to RTF specification, which allows omission of the group when only:
<fonttbl>  '{' \fonttbl (<fontinfo> | ('{' <fontinfo> '}'))+ '}'