Fixes for flutter web (#234)

* Fix for Attribute object comparison

* Fix for "Unexpected null value" error on web

Clipboard is now supported on web, via a permission request through the browser

Co-authored-by: George Johnson <george@indiespring.com>

lib/src/models/documents/attribute.dart

@@ -193,7 +193,7 @@ class Attribute<T> {
   @override
   bool operator ==(Object other) {
     if (identical(this, other)) return true;
-    if (other is! Attribute<T>) return false;
+    if (other is! Attribute) return false;
     final typedOther = other;
     return key == typedOther.key &&
         scope == typedOther.scope &&

lib/src/widgets/raw_editor.dart

@@ -126,8 +126,7 @@ class RawEditorState extends EditorState
 
   DefaultStyles? _styles;
 
-  final ClipboardStatusNotifier? _clipboardStatus =
+  final ClipboardStatusNotifier _clipboardStatus = ClipboardStatusNotifier();
-      kIsWeb ? null : ClipboardStatusNotifier();
   final LayerLink _toolbarLayerLink = LayerLink();
   final LayerLink _startHandleLayerLink = LayerLink();
   final LayerLink _endHandleLayerLink = LayerLink();
@@ -318,7 +317,7 @@ class RawEditorState extends EditorState
   void initState() {
     super.initState();
 
-    _clipboardStatus?.addListener(_onChangedClipboardStatus);
+    _clipboardStatus.addListener(_onChangedClipboardStatus);
 
     widget.controller.addListener(() {
       _didChangeTextEditingValue(widget.controller.ignoreFocusOnTextChange);
@@ -438,8 +437,9 @@ class RawEditorState extends EditorState
     widget.focusNode.removeListener(_handleFocusChanged);
     _focusAttachment!.detach();
     _cursorCont.dispose();
-    _clipboardStatus?.removeListener(_onChangedClipboardStatus);
+    _clipboardStatus
-    _clipboardStatus?.dispose();
+      ..removeListener(_onChangedClipboardStatus)
+      ..dispose();
     super.dispose();
   }
 
@@ -518,7 +518,7 @@ class RawEditorState extends EditorState
         this,
         DragStartBehavior.start,
         null,
-        _clipboardStatus!,
+        _clipboardStatus,
       );
       _selectionOverlay!.handlesVisible = _shouldShowSelectionHandles();
       _selectionOverlay!.showHandles();