Crash when saving with a different name and filetype

[Nico]

I am using Pinta 2.1.1-1 on ArchLinux and it crashes when:
* Open a PNG
* Click Save as
* Change folder (maybe not needed)
* Select jpg as file type
* Click on an existing file (to get a similar name)
* Double click on the filename, to edit/change it
* The app will crash

This is the log
```

(pinta:50651): GdkPixbuf-CRITICAL **: 08:43:26.739: gdk_pixbuf_calculate_rowstride: assertion 'width > 0' failed

(pinta:50651): GdkPixbuf-CRITICAL **: 08:43:26.739: gdk_pixbuf_calculate_rowstride: assertion 'width > 0' failed

(pinta:50651): Gtk-CRITICAL **: 08:43:37.013: gtk_tree_view_scroll_to_cell: assertion 'tree_view->priv->tree != NULL' failed

(pinta:50651): Gtk-CRITICAL **: 08:43:37.030: gtk_tree_view_scroll_to_cell: assertion 'tree_view->priv->tree != NULL' failed

(pinta:50651): Gtk-CRITICAL **: 08:43:37.060: gtk_tree_view_scroll_to_cell: assertion 'tree_view->priv->tree != NULL' failed

(pinta:50651): Gtk-CRITICAL **: 08:43:37.081: gtk_tree_view_scroll_to_cell: assertion 'tree_view->priv->tree != NULL' failed

(pinta:50651): Gtk-CRITICAL **: 08:43:37.098: gtk_tree_view_scroll_to_cell: assertion 'tree_view->priv->tree != NULL' failed

(pinta:50651): Gtk-CRITICAL **: 08:43:37.114: gtk_tree_view_scroll_to_cell: assertion 'tree_view->priv->tree != NULL' failed
**
Gtk:ERROR:../gtk/gtk/gtkrbtree.c:471:_gtk_rbtree_insert_after: assertion failed: (_gtk_rbtree_is_nil (tree->root))
Bail out! Gtk:ERROR:../gtk/gtk/gtkrbtree.c:471:_gtk_rbtree_insert_after: assertion failed: (_gtk_rbtree_is_nil (tree->root))
Aborted (core dumped)
```

Here you can see this crash in action:
https://youtu.be/cj36_p2nKH4

[Cameron White (cameronwhite91)]

I haven't been able to reproduce it so far in my testing, but this likely is an issue in GTK itself since the standard native file dialog is being used. With the tree view-related errors, the crash could possibly depend on the specific folder structure that's being displayed?