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