:bug: Fix npe

kernel/model/blockinfo.go

@@ -253,7 +253,7 @@ func GetBlockIndex(id string) (ret int) {
 	}
 
 	rootChild := node
-	for ; nil == rootChild.Parent || ast.NodeDocument != rootChild.Parent.Type; rootChild = rootChild.Parent {
+	for ; nil != rootChild.Parent && ast.NodeDocument != rootChild.Parent.Type; rootChild = rootChild.Parent {
 	}
 
 	ast.Walk(tree.Root, func(n *ast.Node, entering bool) ast.WalkStatus {