https://github.com/DaveDavenport/gpx-viewer/commit/c8664e489c7d3982420a505d2bebfbb3bfe8e250

Index: libgpx/gpx-viewer-path-layer.vala
--- libgpx/gpx-viewer-path-layer.vala.orig
+++ libgpx/gpx-viewer-path-layer.vala
@@ -256,7 +256,11 @@ namespace Gpx.Viewer
 		ulong view_relocate_handler_id = 0U;
 		ulong view_latitude_handler_id = 0U;
 
+#if CHAMPLAIN_0_12_21
+		public override void set_view (Champlain.View? view)
+#else
 		public override void set_view (Champlain.View view)
+#endif
 		{
 			if (this.view != null) {
 				this.view.disconnect (view_relocate_handler_id);
