treeview: fix a critical warning
treeview: fix a critical warning

gtk_tree_view_column_unset_tree_view() resets column->priv->tree_view to
NULL.
The function is called when a column is removed, but later from the same
function we would call _gtk_tree_view_column_unrealize_button(), which
expects column->priv->tree_view to be != NULL, causing these critical
warnings

Gtk-CRITICAL **: gtk_widget_unregister_window: assertion
`GTK_IS_WIDGET (widget)' failed

This commit moves the call to unset the tree view after the button is
unrealized.

https://bugzilla.gnome.org/show_bug.cgi?id=695473

grubersjoe/adwaita,alexlarsson/gtk,chergert/gtk,jessevdk/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,jigpu/gtk,ahodesuka/gtk,davidgumberg/gtk,msteinert/gtk,jadahl/gtk,jessevdk/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,jadahl/gtk,Distrotech/gtk2,Adamovskiy/gtk,Adamovskiy/gtk,chergert/gtk,bratsche/gtk-,jadahl/gtk,Lyude/gtk-,ebassi/gtk,ahodesuka/gtk,bratsche/gtk-,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,ebassi/gtk,chergert/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,chergert/gtk,ebassi/gtk,Distrotech/gtk2,ahodesuka/gtk,jadahl/gtk,ahodesuka/gtk,jigpu/gtk,grubersjoe/adwaita,Lyude/gtk-,Lyude/gtk-,jadahl/gtk,Adamovskiy/gtk,Adamovskiy/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,bratsche/gtk-,davidgumberg/gtk,jigpu/gtk,jessevdk/gtk,grubersjoe/adwaita,ahodesuka/gtk,davidgumberg/gtk,jessevdk/gtk,davidgumberg/gtk,msteinert/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,jessevdk/gtk,msteinert/gtk,Lyude/gtk-,Lyude/gtk-,bratsche/gtk-,msteinert/gtk,ebassi/gtk,davidgumberg/gtk,chergert/gtk,davidgumberg/gtk,Adamovskiy/gtk,alexlarsson/gtk,jigpu/gtk,msteinert/gtk,ahodesuka/gtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Distrotech/gtk2,chergert/gtk,ahodesuka/gtk,Distrotech/gtk2,bratsche/gtk-,jadahl/gtk,alexlarsson/gtk,ebassi/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,bratsche/gtk-,jadahl/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,ebassi/gtk,jessevdk/gtk,Distrotech/gtk2,grubersjoe/adwaita,alexlarsson/gtk,Lyude/gtk-,Distrotech/gtk2