issue-149 add null check to second pass of list block instead of non null assertion

Author: cnamqui

src/index/import/markdown.ts

@@ -188,7 +188,7 @@ export function markdownSourceImport(
     // In the second list pass, actually construct the list heirarchy.
     for (const item of listItems.values()) {
         if (item.parentLine < 0) {
-            const listBlock = blocks.getPairOrNextHigher(-item.parentLine - 1)![1];
+            const listBlock = blocks.getPairOrNextHigher(-item.parentLine - 1)?.[1];
             if (!listBlock || !(listBlock.type === "list")) continue;
 
             (listBlock as ListBlockData).items.push(item);