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