Fixes Dolphin crash on "Defocus Terminal Panel" menu click if no Konsole is installed.

Summary:
Dolphin crashes if no Konsole is installed and user clicks menu action "Defocus Terminal Panel".
This fix is pretty straight forward.

Steps to reproduce:
1. Run Dolphin without Konsole available.
2. Press F4 to open console window.
3. Click in service menu "Focus Terminal Panel".
4. Click in service menu "Defocus Terminal Panel". Observe the crash.

Test Plan:
1. Run Dolphin without Konsole available.
2. Press F4 to open console window.
3. Click in service menu "Focus Teminal Panel".
4. Click in service menu "Defocus Terminal Panel".
5. Click in service menu "Focus Terminal Panel".
6. Press F4 to close console window.

Reviewers: #dolphin, meven, elvisangelaccio, ngraham

Reviewed By: #dolphin, meven, elvisangelaccio, ngraham

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D29345