Added `unknownEmbedBuilder` to QuillEditor (#1073)

* Added `unknownEmbedBuilder` to QuillEditor

* `unknownEmbedBuilder` constructor parameter

lib/src/widgets/editor.dart

@@ -173,6 +173,7 @@ class QuillEditor extends StatefulWidget {
       this.onSingleLongTapMoveUpdate,
       this.onSingleLongTapEnd,
       this.embedBuilders,
+      this.unknownEmbedBuilder,
       this.linkActionPickerDelegate = defaultLinkActionPickerDelegate,
       this.customStyleBuilder,
       this.locale,
@@ -359,6 +360,7 @@ class QuillEditor extends StatefulWidget {
       onSingleLongTapEnd;
 
   final Iterable<EmbedBuilder>? embedBuilders;
+  final EmbedsBuilder? unknownEmbedBuilder;
   final CustomStyleBuilder? customStyleBuilder;
 
   /// The locale to use for the editor toolbar, defaults to system locale
@@ -491,7 +493,13 @@ class QuillEditorState extends State<QuillEditor>
         node,
         readOnly,
       ) =>
-          _buildCustomBlockEmbed(node, context, controller, readOnly),
+          _buildCustomBlockEmbed(
+            node,
+            context,
+            controller,
+            readOnly,
+            widget.unknownEmbedBuilder,
+          ),
       linkActionPickerDelegate: widget.linkActionPickerDelegate,
       customStyleBuilder: widget.customStyleBuilder,
       floatingCursorDisabled: widget.floatingCursorDisabled,
@@ -525,18 +533,22 @@ class QuillEditorState extends State<QuillEditor>
     return editor;
   }
 
-  Widget _buildCustomBlockEmbed(Embed node, BuildContext context,
+  Widget _buildCustomBlockEmbed(
-      QuillController controller, bool readOnly) {
+    Embed node,
+    BuildContext context,
+    QuillController controller,
+    bool readOnly,
+    EmbedsBuilder? unknownEmbedBuilder,
+  ) {
     final builders = widget.embedBuilders;
     
-    if (builders != null) {
     var _node = node;
-
     // Creates correct node for custom embed
     if (node.value.type == BlockEmbed.customType) {
       _node = Embed(CustomBlockEmbed.fromJsonString(node.value.data));
     }
 
+    if (builders != null) {
       for (final builder in builders) {
         if (builder.key == _node.value.type) {
           return builder.build(context, controller, _node, readOnly);
@@ -544,10 +556,15 @@ class QuillEditorState extends State<QuillEditor>
       }
     }
     
+    if (unknownEmbedBuilder != null) {
+      return unknownEmbedBuilder(context, controller, _node, readOnly);
+    }
+
     throw UnimplementedError(
       'Embeddable type "${node.value.type}" is not supported by supplied '
       'embed builders. You must pass your own builder function to '
-      'embedBuilders property of QuillEditor or QuillField widgets.',
+      'embedBuilders property of QuillEditor or QuillField widgets or '
+      'specify an unknownEmbedBuilder.',
     );
   }