Dear Developers,

A drag-and-drop of a tab onto the document area causes Mousepad (0.4) to crash under Ubuntu MATE 18.04. Below is the terminal output:

(mousepad:6329): Gtk-WARNING **: 12:59:04.113: Theme parsing error: <data>:2:29: The style property GtkButton:default-border is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:6329): Gtk-WARNING **: 12:59:04.113: Theme parsing error: <data>:3:37: The style property GtkButton:default-outside-border is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:6329): Gtk-WARNING **: 12:59:04.113: Theme parsing error: <data>:4:27: The style property GtkButton:inner-border is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:6329): Gtk-WARNING **: 12:59:04.113: Theme parsing error: <data>:5:31: The style property GtkWidget:focus-line-width is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:6329): Gtk-WARNING **: 12:59:04.113: Theme parsing error: <data>:6:28: The style property GtkWidget:focus-padding is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:6329): GLib-GObject-CRITICAL **: 12:59:07.939: g_object_ref: assertion 'G_IS_OBJECT (object)' failed

(mousepad:6329): Gtk-CRITICAL **: 12:59:07.939: gtk_container_get_focus_child: assertion 'GTK_IS_CONTAINER (container)' failed

(mousepad:6329): GLib-GObject-CRITICAL **: 12:59:07.939: g_object_unref: assertion 'G_IS_OBJECT (object)' failed

(mousepad:6329): Gtk-CRITICAL **: 12:59:07.939: gtk_container_get_focus_child: assertion 'GTK_IS_CONTAINER (container)' failed
**
Gtk:ERROR:../../../../gtk/gtkwidget.c:5823:gtk_widget_get_frame_clock: assertion failed: (window != NULL)
Aborted (core dumped)

Thank you