https://github.com/pdfpc/pdfpc/issues/686
https://bugs.gentoo.org/904970

commit 18beaecbbcc066e0d4c889b3aa3ecaa7351f7768
Author: Evgeny Stambulchik <fnevgeny@gmail.com>
Date:   Tue, 25 Apr 2023 16:11:25 +0300

    Create Lists of nullable types

--- a/src/classes/drawings/drawing_commands.vala
+++ b/src/classes/drawings/drawing_commands.vala
@@ -54,8 +54,8 @@
         }
 
         public void clear() {
-            this.drawing_commands = new List<DrawingCommand>();
-            this.redo_commands = new List<DrawingCommand>();
+            this.drawing_commands = new List<DrawingCommand?>();
+            this.redo_commands = new List<DrawingCommand?>();
         }
 
         public void add_line(bool is_eraser,
@@ -70,7 +70,7 @@
 
             // After adding a new line you can no longer redo the old
             // path.
-            this.redo_commands = new List<DrawingCommand>(); // clear
+            this.redo_commands = new List<DrawingCommand?>(); // clear
 
             bool new_path = true;
             double epsilon = 1e-4; // Less than 0.1 pixel for a 1000x1000 img