Crash when pressing PgDn/PgUp in gradient dialog

Migrated from: https://bugs.launchpad.net/inkscape/+bug/1392668

Was reproduced on XP and Ubuntu, can be reproduced since 0.91 and still reproduced on 0.92.4 and master.

Steps to reproduce:

1. Create a shape (e.g. rectangle).
2. Change fill to linear gradient.
3. Create a duplicate gradient (click + button).
4. Mouse click below gradient name (white space inside gradient list).
5. Press PgDn, then press PgUp.

What happened?

Inkscape crashes with following message:

Gtk:ERROR:gtkrbtree.c:967:_gtk_rbtree_node_find_offset: assertion failed: (node->left)

What should have happened?

No crash should have occurred.

Inkscape Version and Operating System:

• Inkscape Version: 1.0alpha (75da9ac4, 2019-04-12)
• Operating System: openSUSE Tumbleweed
• Operating System version: 20190411