KCM/ComponentChooser Treat cases when there is no app for a usage

Summary: BUG: 417276

Test Plan:
Have no email client installed.
In kcm componentchooser
 - select another browser
 - Save

Before:
 - Crash
After:
 - No crash

Reviewers: bport, ervin, crossi, davidedmundson, #plasma

Reviewed By: bport, ervin, crossi

Subscribers: anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27395