MyPaint crashing with group layer creation

[americogobbo]

Description of the problem

MyPaint is crashing when you try to create a Group Layer. The issue happens always before or after you create a single layer. For instance, the Mypaint opens directly with a layer by default, if you create a group layer after the program crash.

Basic system details

MyPaint version: 2.0.1+gitexport.478280f

Operating system: [Fedora 39]
Desktop environment: [GNOME 45]

Steps to reproduce

1. Start MyPaint
2. Create a Group layer
3. Crash

Backtraces or error messages

Mypaint version: 2.0.1+gitexport.478280f
System information: Linux-6.5.10-300.fc39.x86_64-x86_64-with-glibc2.38
Using: Python 3.12.0, GTK 3.24.38, GdkPixbuf 2.42.10, Cairo 1.18.0, GLib 2.78.0
Traceback (most recent call last):
  File "/usr/lib64/mypaint/gui/document.py", line 1404, new_layer_cb(self=<gui.document.Document object>, action=<Gtk.Action object at 0x7f356e29aa00 (GtkAction at 0x5610150d13c0)>)
            self.model.add_layer(path, layer_class=layer_class, **layer_kwds)
            self.blink_layer(action)
  variables: {'self.model.add_layer': ('local', <bound method Document.add_layer of <Document nlayers=2 bbox=Rect(0, 0, 0, 0) paintonly=False>>), 'path': ('local', (0,)), 'layer_class': ('local', <class 'lib.layer.group.LayerStack'>), 'layer_kwds': ('local', {})}
  File "/usr/lib64/mypaint/lib/document.py", line 1286, add_layer(self=<Document nlayers=2 bbox=Rect(0, 0, 0, 0) paintonly=False>, path=(0,), layer_class=<class 'lib.layer.group.LayerStack'>, **kwds={})
            )
            self.do(cmd)
  variables: {'self.do': ('local', <bound method Document.do of <Document nlayers=2 bbox=Rect(0, 0, 0, 0) paintonly=False>>), 'cmd': ('local', <Add Group>)}
  File "/usr/lib64/mypaint/lib/document.py", line 1161, do(self=<Document nlayers=2 bbox=Rect(0, 0, 0, 0) paintonly=False>, cmd=<Add Group>)
            self.sync_pending_changes()
            self.command_stack.do(cmd)
  variables: {'self.command_stack.do': ('local', <bound method CommandStack.do of <CommandStack undo_len=0 redo_len=0>>), 'cmd': ('local', <Add Group>)}
  File "/usr/lib64/mypaint/lib/command.py", line 75, do(self=<CommandStack undo_len=0 redo_len=0>, command=<Add Group>)
            self._discard_redo()
            completed = command.redo()
            # NOTE: The below is not equivalent to checking truthiness,
  variables: {'completed': (None, []), 'command.redo': ('local', <bound method AddLayer.redo of <Add Group>>)}
  File "/usr/lib64/mypaint/lib/command.py", line 951, redo(self=<Add Group>)
            self._prev_currentlayer_path = layers.get_current_path()
            layers.deepinsert(self._insert_path, self._layer)
            assert self._layer.name is not None
  variables: {'layers.deepinsert': ('local', <bound method RootLayerStack.deepinsert of <RootLayerStack len=2>>), 'self._insert_path': ('local', (0,)), 'self._layer': ('local', <LayerStack len=0 'Group'>)}
  File "/usr/lib64/mypaint/lib/layer/tree.py", line 1718, deepinsert(self=<RootLayerStack len=2>, path=(0,), layer=<LayerStack len=0 'Group'>)
                    stack.insert(idx, layer)
                    layer.name = self.get_unique_name(layer)
                    self._propagate_symmetry_state(layer)
  variables: {'layer.name': ('local', 'Group'), 'self.get_unique_name': ('local', <bound method RootLayerStack.get_unique_name of <RootLayerStack len=2>>), 'layer': ('local', <LayerStack len=0 'Group'>)}
  File "/usr/lib64/mypaint/lib/layer/core.py", line 335, name(self=<LayerStack len=0 'Group'>, name='Group')
            if self._name != oldname:
                self._properties_changed(["name"])
  variables: {'self._properties_changed': ('local', <bound method LayerBase._properties_changed of <LayerStack len=0 'Group'>>)}
  File "/usr/lib64/mypaint/lib/layer/core.py", line 528, _properties_changed(self=<LayerStack len=0 'Group'>, properties=['name'])
            if root is not None:
                root._notify_layer_properties_changed(self, set(properties))
  variables: {'root._notify_layer_properties_changed': ('local', <bound method RootLayerStack._notify_layer_properties_changed of <RootLayerStack len=2>>), 'self': ('local', <LayerStack len=0 'Group'>), 'set': ('builtin', <class 'set'>), 'properties': ('local', ['name'])}
  File "/usr/lib64/mypaint/lib/layer/tree.py", line 2590, _notify_layer_properties_changed(self=<RootLayerStack len=2>, layer=<LayerStack len=0 'Group'>, changed={'name'})
            assert path is not None, "Unable to find layer which was changed"
            self.layer_properties_changed(path, layer, changed)
  variables: {'self.layer_properties_changed': ('local', <_MethodWithObservers lib.layer.tree.RootLayerStack.layer_properties_changed>), 'path': ('local', (0,)), 'layer': ('local', <LayerStack len=0 'Group'>), 'changed': ('local', {'name'})}
  File "/usr/lib64/mypaint/lib/observable.py", line 289, __call__(self=<_MethodWithObservers lib.layer.tree.RootLayerStack.layer_properties_changed>, *args=((0,), <LayerStack len=0 'Group'>, {'name'}), **kwargs={})
                    try:
                        observer(observed, *args, **kwargs)
                    except _BoundObserverMethod._ReferenceError:
  variables: {'observer': ('local', <_BoundObserverMethod gui.tileddrawwidget.CanvasRenderer.layer_props_changed_cb>), 'observed': (None, []), 'args': ('local', ((0,), <LayerStack len=0 'Group'>, {'name'})), 'kwargs': ('local', {})}
  File "/usr/lib64/mypaint/lib/observable.py", line 474, __call__(self=<_BoundObserverMethod gui.tileddrawwidget.CanvasRenderer.layer_props_changed_cb>, observed=<RootLayerStack len=2>, *args=((0,), <LayerStack len=0 'Group'>, {'name'}), **kwargs={})
                raise self._ReferenceError
            self._observer_func(observer, observed, *args, **kwargs)
            del observer
  variables: {'self._observer_func': ('local', <function CanvasRenderer.layer_props_changed_cb at 0x7f356e3ff420>), 'observer': ('local', <tileddrawwidget.CanvasRenderer object at 0x7f356e27acc0 (gui+tileddrawwidget+CanvasRenderer at 0x5610151d4c10)>), 'observed': ('local', <RootLayerStack len=2>), 'args': ('local', ((0,), <LayerStack len=0 'Group'>, {'name'})), 'kwargs': ('local', {})}
  File "/usr/lib64/mypaint/gui/tileddrawwidget.py", line 933, layer_props_changed_cb(self=<tileddrawwidget.CanvasRenderer object at 0x7f35...ileddrawwidget+CanvasRenderer at 0x5610151d4c10)>, rootstack=<RootLayerStack len=2>, path=(0,), layer=<LayerStack len=0 'Group'>, changed={'name'})
        def layer_props_changed_cb(self, rootstack, path, layer, changed):
            self.update_cursor()
  variables: {'self.update_cursor': ('local', <bound method DrawCursorMixin.update_cursor of <tileddrawwidget.CanvasRenderer object at 0x7f356e27acc0 (gui+tileddrawwidget+CanvasRenderer at 0x5610151d4c10)>>)}
  File "/usr/lib64/mypaint/gui/tileddrawwidget.py", line 590, update_cursor(self=<tileddrawwidget.CanvasRenderer object at 0x7f35...ileddrawwidget+CanvasRenderer at 0x5610151d4c10)>)
                # Often a red circle with a diagonal bar through it.
                c = Gdk.Cursor.new_for_display(
                    window.get_display(), Gdk.CursorType.CIRCLE)
  variables: {'c': (None, []), 'Gdk.Cursor.new_for_display': ('global', gi.FunctionInfo(new_for_display, bound=<class 'gi.overrides.Gdk.Cursor'>)), 'window.get_display': ('local', gi.FunctionInfo(get_display, bound=<__gi__.GdkWaylandWindow object at 0x7f356c771100 (GdkWaylandWindow at 0x561015de4020)>)), 'Gdk.CursorType.CIRCLE': ('global', <enum GDK_CIRCLE of type Gdk.CursorType>)}
TypeError: constructor returned NULL
```

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