fix: use default branch

lib/screens/object.dart

@@ -22,7 +22,7 @@ class ObjectScreen extends StatelessWidget {
   ObjectScreen({
     @required this.owner,
     @required this.name,
-    this.branch = 'master',
+    @required this.branch,
     this.paths = const [],
     this.type = 'tree',
   });

lib/screens/repo.dart

@@ -240,7 +240,11 @@ class RepoScreen extends StatelessWidget {
                     ),
                   ],
                 ),
-                screenBuilder: (_) => ObjectScreen(owner: owner, name: name),
+                screenBuilder: (_) => ObjectScreen(
+                  owner: owner,
+                  name: name,
+                  branch: payload['defaultBranchRef']['name'], // FIXME: null
+                ),
               ),
               TableViewItem(
                 leftWidget: Icon(Octicons.issue_opened, size: 20),