Fix crash on failed opening of query in Design view

Summary:
For D17336.

Branch: 393094-date-constants

Test Plan:

• Right click on query and select Design
• Crash when prompted that cannot open in design view, select option to open in text view

https://phabricator.kde.org/D17336#382812

Development note: schema object is deleted automatically if owned by the KexiWindow, otherwise no need to delete.

FIXED-IN:3.2.0

T10134

Reviewers: piggz

Reviewed By: piggz

Subscribers: Kexi-Devel-list

Tags: KEXI

Maniphest Tasks: T10134

Differential Revision: https://phabricator.kde.org/D17847