We deactivate the previously active view container whenever we change the active
split view, but we never do the same when we close the split view.
Long term we should probably even delete the secondary view after
closing the split view, because it will never be used again and the
pointer will be overwritten the next time the user opens the split view.
For now it contains a test case for the crash in bug #379135
CCBUG: 379135
Reviewers: emmanuelp, dfaure
Differential Revision: https://phabricator.kde.org/D5936