[shotwell] Fix non-matching generic type ownership

[Rico Tzschichholz <ricotz src gnome org>]

commit dec6659fa025ad278da1570bf63f3ec64e55f577
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Sun Feb 24 19:38:21 2019 +0100

    Fix non-matching generic type ownership

 src/MapWidget.vala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/MapWidget.vala b/src/MapWidget.vala
index 3959649f..f929f957 100644
--- a/src/MapWidget.vala
+++ b/src/MapWidget.vala
@@ -107,8 +107,8 @@ private class DataViewPositionMarker : PositionMarker {
 }
 
 private class MarkerGroup : PositionMarker {
-    private Gee.Collection<DataViewPositionMarker> _data_view_position_markers =
-        new Gee.LinkedList<DataViewPositionMarker>();
+    private Gee.Collection<weak DataViewPositionMarker> _data_view_position_markers =
+        new Gee.LinkedList<weak DataViewPositionMarker>();
     private Gee.Collection<PositionMarker> _position_markers = new Gee.LinkedList<PositionMarker>();
     private Champlain.BoundingBox bbox = new Champlain.BoundingBox();