[Flutter] Force a username before subscribing

flutter/lib/utils/dialogs.dart

@@ -28,6 +28,40 @@ class UIDialogs {
     );
   }
 
+  static Future<String?> showUsernameRequiredDialog(BuildContext context) {
+    var _textFieldController = TextEditingController();
+
+    return showDialog(
+      context: context,
+      builder: (context) => AlertDialog(
+        title: Text('Username Required'),
+        content: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Text('Please set a public username to subscribe to channels from other users.'),
+            SizedBox(height: 16),
+            TextField(
+              autofocus: true,
+              controller: _textFieldController,
+              decoration: InputDecoration(hintText: 'Enter username'),
+            ),
+          ],
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.of(context).pop(),
+            child: Text('Cancel'),
+          ),
+          TextButton(
+            onPressed: () => Navigator.of(context).pop(_textFieldController.text),
+            child: Text('OK'),
+          ),
+        ],
+      ),
+    );
+  }
+
   static Future<bool> showConfirmDialog(BuildContext context, String title, {String? text, String? okText, String? cancelText}) {
     return showDialog<bool>(
       context: context,