make Timeline::set_tracks(null_ptr) create a new Stack to fix crash (#961)

* make Timeline::set_tracks(null_ptr) create a new Stack, just like
the constructor does

Co-authored-by: David Baraff <nosuch@nowhere>
Co-authored-by: Nick Porcino <[email protected]>

Author: 3 people

src/opentimelineio/timeline.cpp

@@ -13,6 +13,10 @@ Timeline::Timeline(std::string const& name,
 Timeline::~Timeline() {
 }
 
+void Timeline::set_tracks(Stack* stack) {
+    _tracks = stack ? stack : new Stack("tracks");
+}
+
 bool Timeline::read_from(Reader& reader) {
     return reader.read("tracks", &_tracks) &&
         reader.read_if_present("global_start_time", &_global_start_time) &&

src/opentimelineio/timeline.h

@@ -30,9 +30,7 @@ class Timeline : public SerializableObjectWithMetadata {
     }*/
 
 
-    void set_tracks(Stack* stack) {
-        _tracks = stack;
-    }
+    void set_tracks(Stack* stack);
 
     optional<RationalTime> const& global_start_time() const {
         return _global_start_time;

tests/test_timeline.py

@@ -494,6 +494,24 @@ def test_tracks(self):
             [t.name for t in tl.audio_tracks()]
         )
 
+    def test_tracks_set_null_tracks(self):
+        tl = otio.schema.Timeline(tracks=[
+            otio.schema.Track(
+                name="V1",
+                kind=otio.schema.TrackKind.Video
+            ),
+            otio.schema.Track(
+                name="V2",
+                kind=otio.schema.TrackKind.Video
+            )])
+
+        self.assertEqual(len(tl.tracks), 2)
+        self.assertTrue(isinstance(tl.tracks, otio.schema.Stack))
+        tl.tracks = None
+        self.assertEqual(len(tl.audio_tracks()), 0)
+        self.assertEqual(len(tl.video_tracks()), 0)
+        self.assertTrue(isinstance(tl.tracks, otio.schema.Stack))
+
 
 if __name__ == '__main__':
     unittest.main()