Created attachment 120367 [details]
Sample xmi file to show problem - but happens with any file.

SUMMARY
Cannot open any files. Files can be saved.

STEPS TO REPRODUCE
1. Open umbrello
2. Open file

OBSERVED RESULT
Error dialog shows up: The file <file:///....> does not exist

EXPECTED RESULT
File should open

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Fedora Release 29; Umbrello: 2.27.2 / Fedora package: umbrello-18.12.2-1.fc29.src.rpm
(available in About System):
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Error dialog is thrown in line 517 of umldoc.cpp as tmpfile.exists() (in line 512) returns false

I can use gdb to manually jump from 512 to 537 and am able to open files.
To reproduce...
 1. Install debuginfo files for umbrello (and qt5-core)
 2. Break at *0x000055555565b37b (break *0x000055555565b37b)
 3. Jump past if block (j *0x55555565b396)

For some reason tmpfile::exists() suddenly reports false in line 512. file.exists() reports true in the same line!

I reported the bug to redhat/fedora (https://bugzilla.redhat.com/show_bug.cgi?id=1711446) - but they requested that it be filed here also/instead.