fix: deeply nested lists don't throw an error

Fixes #583

src/examples/nested-lists.tsx

@@ -0,0 +1,32 @@
+import React from 'react'
+import { MDXEditor, listsPlugin, diffSourcePlugin, toolbarPlugin, DiffSourceToggleWrapper, UndoRedo } from '..'
+
+const listsMarkdown = `
+* hello
+* world
+    * indented
+  * more
+* back
+`
+
+export function NestedLists() {
+  return (
+    <MDXEditor
+      markdown={listsMarkdown}
+      onChange={(md) => {
+        console.log(md)
+      }}
+      plugins={[
+        listsPlugin(),
+        diffSourcePlugin(),
+        toolbarPlugin({
+          toolbarContents: () => (
+            <DiffSourceToggleWrapper>
+              <UndoRedo />
+            </DiffSourceToggleWrapper>
+          )
+        })
+      ]}
+    />
+  )
+}

src/plugins/lists/LexicalListItemVisitor.ts

@@ -10,8 +10,15 @@ export const LexicalListItemVisitor: LexicalExportVisitor<ListItemNode, Mdast.Li
 
     if (children.length === 1 && $isListNode(firstChild)) {
       // append the list after the paragraph of the previous list item
-      const prevListItemNode = mdastParent.children.at(-1) as Mdast.ListItem
-      actions.visitChildren(lexicalNode, prevListItemNode)
+      const prevListItemNode = mdastParent.children.at(-1) as Mdast.ListItem | undefined
+      // XXX: this is a hack to avoid errors with deeply nested lists - the approach will flatten them.
+      // Deeply nested lists are still not supported, but at least no error will be thrown.
+      if (!prevListItemNode) {
+        // @ts-expect-error - MDAST type is incorrect
+        actions.visitChildren(firstChild, mdastParent)
+      } else {
+        actions.visitChildren(lexicalNode, prevListItemNode)
+      }
     } else {
       const parentList = lexicalNode.getParent()! as ListNode
       // nest the children in a paragraph for MDAST compatibility