Fix crash when removing toolview via context menu

Summary:
When using "Remove Toolview" from the toolview context menus, kdevelop would crash.
Postponing the button deletion seems to fix the problem.

Test Plan: Remove a toolview from the context menu without crash now.

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6261