Fix this bug reported by Mael Hilléreau:

Fix this bug reported by Mael Hilléreau:

1. Open a document with child documents, each comprising divisions;
2. Show the outline pane. You can see childs' divisions;
3. Select (view) one of the childs;
4. Close this child doc. The focus comes back to the master doc;
5. Click in the outline pane at some point corresponding to the child doc you closed. LyX crashes with SIGSEGV (instead of reopening the child...).

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18939 a592a061-630c-0410-9148-cb99ea01b6c8