Fix crash when global search box is used after re-opening another project

Summary:
(remove search models from the search box before destruction)

FIXED-IN:3.1.0

Test Plan:

1. Open a project
2. Try to use the search box by entering something

Expected: searching works

3. Close the project without closing KEXI
4. Repeat step 2

Expected: nothing is found because project is not open (previously there were crashes here)

5. (Test if opening another project works OK): Open other project in the same KEXI instance
6. Repeat step 2

Expected: searching works

7. Repeat step 4

Expected: nothing is found (no crashes)

8. Close KEXI

Expected: clean exit, no crashes

Reviewers: piggz

Reviewed By: piggz

Tags: KEXI

Differential Revision: https://phabricator.kde.org/D10963