support for custom non-styling attribute

.vscode/launch.json

@@ -0,0 +1,15 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "flutter-quill",
+            "request": "launch",
+            "type": "dart",
+            "program": "app/lib/main.dart"
+        },
+       
+    ]
+}

lib/models/documents/attribute.dart

@@ -155,7 +155,8 @@ class Attribute<T> {
 
   static Attribute fromKeyValue(String key, dynamic value) {
     if (!_registry.containsKey(key)) {
-      throw ArgumentError.value(key, 'key "$key" not found.');
+       return null;
+      // throw ArgumentError.value(key, 'key "$key" not found.');
     }
     Attribute origin = _registry[key];
     Attribute attribute = clone(origin, value);

lib/models/documents/style.dart

@@ -17,7 +17,7 @@ class Style {
 
     Map<String, Attribute> result = attributes.map((String key, dynamic value) {
       Attribute attr = Attribute.fromKeyValue(key, value);
-      return MapEntry<String, Attribute>(key, attr);
+      return MapEntry<String, Attribute>(key, attr??Attribute(key, AttributeScope.INLINE, value));
     });
     return Style.attr(result);
   }

lib/widgets/toolbar.dart

@@ -742,6 +742,7 @@ class _HistoryButtonState extends State<HistoryButton> {
   }
 
   void _setIconColor() {
+    if(!mounted) return;
     if (widget.undo) {
       setState(() {
         _iconColor = widget.controller.hasUndo