store [nfc]: Add PerAccountStoreWidget.maybeOf

Author: chrisbobbe

lib/widgets/store.dart

@@ -203,6 +203,8 @@ class PerAccountStoreWidget extends StatefulWidget {
   /// on the server.
   ///
   /// See also:
+  ///  * [maybeOf], for a version of this that returns null instead of erroring
+  ///    if there is no [PerAccountStoreWidget] ancestor.
   ///  * [accountIdOf], for the account ID corresponding to the same data.
   ///  * [GlobalStoreWidget.of], for the app's data beyond that of a
   ///    particular account.
@@ -213,6 +215,21 @@ class PerAccountStoreWidget extends StatefulWidget {
     return widget!.store;
   }
 
+  /// Like [of], but returns null instead of erroring if there is no
+  /// [PerAccountStoreWidget] ancestor.
+  ///
+  /// Useful in particular for content widgets that have special functionality
+  /// when they appear in a per-account context, which is the common case
+  /// (the message list).
+  ///
+  /// This will be useful for #488, supporting Zulip content rendering
+  /// in e.g. an org description shown before you log in.
+  // TODO(#488) Remove #488 comment.
+  static PerAccountStore? maybeOf(BuildContext context) {
+    final widget = context.dependOnInheritedWidgetOfExactType<_PerAccountStoreInheritedWidget>();
+    return widget?.store;
+  }
+
   /// Our account ID for the relevant account for this widget.
   ///
   /// As with [of], the data is taken from the closest [PerAccountStoreWidget]