[super_editor_markdown] Fix compilation error (#2406)

super_editor_markdown/lib/src/super_editor_paste_markdown.dart

@@ -40,15 +40,19 @@ Future<void> pasteMarkdown({
   required Document document,
   required DocumentComposer composer,
 }) async {
-  DocumentPosition pastePosition = composer.selection!.extent;
-
   // Delete all currently selected content.
   if (!composer.selection!.isCollapsed) {
-    pastePosition = CommonEditorOperations.getDocumentPositionAfterExpandedDeletion(
+    final pastePosition = CommonEditorOperations.getDocumentPositionAfterExpandedDeletion(
       document: document,
       selection: composer.selection!,
     );
 
+    if (pastePosition == null) {
+      // A null pastePosition means that the selection can't be deleted. This might happen
+      // when the selection contains only non-deletable nodes. Therefore, we cannot paste.
+      return;
+    }
+
     // Delete the selected content.
     editor.execute([
       DeleteContentRequest(documentRange: composer.selection!),
@@ -67,7 +71,7 @@ Future<void> pasteMarkdown({
   editor.execute([
     PasteStructuredContentEditorRequest(
       content: deserializedMarkdown,
-      pastePosition: pastePosition,
+      pastePosition: composer.selection!.extent,
     ),
   ]);
 }