From PHP 7 is Tracy unexpectedly dependent to php-xml extenstion (is using function utf8_decode().
It is probably problem only with @oerdnj 's PPA packages. Really i don't know if is that fskutečnosti relevant to Tracy, but that PPAs are very popular (only one trust source for builded & updated PHP on Ubuntu) and Nette should be verify availability of extension in composer.json.
I can send PR, please only confirm I am thinking right.