[gimp] app: null the pointer after destruction.

[Jehan <jehanp src gnome org>]

commit 91c69b782ca4aa94217d12b3361648b83e04473f
Author: Jehan <jehan girinstud io>
Date:   Fri Apr 19 09:28:22 2019 +0200

    app: null the pointer after destruction.
    
    Make sure we don't end up with a broken pointer in case it was not
    immediately re-set.

 app/widgets/gimpcolormapeditor.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/app/widgets/gimpcolormapeditor.c b/app/widgets/gimpcolormapeditor.c
index be23a2dc2a..52d1d6dfd0 100644
--- a/app/widgets/gimpcolormapeditor.c
+++ b/app/widgets/gimpcolormapeditor.c
@@ -168,6 +168,7 @@ gimp_colormap_editor_set_context (GimpDocked  *docked,
 
   if (editor->selection)
     gtk_widget_destroy (editor->selection);
+  editor->selection = NULL;
 
   /* Main selection widget. */
   if (context)