Add `tokenOrNull` util function

lib/util/token_or_null.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../stores/accounts_store.dart';
+
+/// returns either raw token for default user of `instanceUrl`
+/// or null if this instance is not added or doesn't
+/// have any user logged in
+String tokenOrNull(BuildContext context, String instanceUrl) {
+  if (context
+      // ignore: avoid_types_on_closure_parameters
+      .select((AccountsStore store) => store.isAnonymousFor(instanceUrl))) {
+    return null;
+  }
+  return context
+      // ignore: avoid_types_on_closure_parameters
+      .select((AccountsStore store) => store.defaultTokenFor(instanceUrl).raw);
+}